ch04
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ch04

Course Number: PHYSICS 201, Fall 2012

College/University: Rutgers

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CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (b) If only one force acts on the object, it is the net force; thus, the net force must be nonzero. Consequently, the velocity would change, according to Newtons first law, and could not be constant. 2. (d) This situation violates the first law, which predicts that the rabbits foot tends to remain in place where it was when...

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4 CHAPTER FORCES AND NEWTON'S LAWS OF MOTION ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (b) If only one force acts on the object, it is the net force; thus, the net force must be nonzero. Consequently, the velocity would change, according to Newtons first law, and could not be constant. 2. (d) This situation violates the first law, which predicts that the rabbits foot tends to remain in place where it was when the car begins accelerating. The car would leave the rabbits foot behind. That is, the rabbits foot would swing away from, not toward, the windshield. 3. (e) Newtons first law states that an object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. All three statements are consistent with the first law. 4. (a) Newtons second law with a net force of 7560 N 7340 N = 220 N due north gives the answer directly. 5. (c) Newtons second law gives the answer directly, provided the net force is calculated by vector addition of the two given forces. The direction of the net force gives the direction of the acceleration. 6. (e) Newtons second law gives the answer directly. One method is to determine the total acceleration by vector addition of the two given components. The net force has the same direction as the acceleration. 7. (e) Answers a and b are false, according to the third law, which states that whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. It does not matter whether one of the bodies is stationary or whether it collapses. Answer c is false, because according to the third law, Sam and his sister experience forces of equal magnitudes during the push-off. Since Sam has the greater mass, he flies off with the smaller acceleration, according to the second law. Answer d is false, because in catching and throwing the ball each astronaut applies a force to it, and, according to the third law, the ball applies an oppositely directed force of equal magnitude to each astronaut. These reaction forces accelerate the astronauts away from each other, so that the distance between them increases. 8. (b) Newtons third law indicates that Paul and Tom apply forces of equal magnitude to each other. According to Newtons second law, the magnitude of each of these forces is the mass 160 FORCES AND NEWTON'S LAWS OF MOTION times the magnitude of the acceleration. Thus, we have mPaulaPaul = mTomaTom, or mPaul/mTom = aTom/aPaul. 9. (e) Newtons law of gravitation gives the answer directly. According to this law the weight is directly proportional to the mass of the planet, so twice the mass means twice the weight. However, this law also indicates that the weight is inversely proportional to the square of the planets radius, so three times the radius means one ninth the weight. Together, these two factors mean that the weight on the planet is 2/9 or 0.222 times your earth-weight. 10. (c) Newtons law of gravitation gives the answer, provided that the distance between the centers of the spheres is used for r (r = 0.50 m + 1.20 m + 0.80 m), rather than the distance between the surfaces of the spheres. 11. (a) The answer follows directly from the fact that weight W is given by W = mg, where m is the mass and g is the acceleration due to the earths gravity. Thus, m = (784 N)/(9.80 m/s2) = 80.0 kg. The mass is the same on Mercury as on Earth, because mass is an intrinsic property of matter. 12. (d) What matters is the direction of the elevators acceleration. When the acceleration is upward, the apparent weight is greater than the true weight. When the acceleration is downward, the apparent weight is less than the true weight. In both possibilities the acceleration points upward. 13. (b) According to Newtons third law, the pusher and the wall exert forces of equal magnitude but opposite directions on each other. The normal force is the component of the walls force that is perpendicular to the wall. Thus, it has the same magnitude as the component of the pushers force that is perpendicular to the wall. As a result, the normal forces are ranked in the same order as the perpendicular components of the pushers forces. The smallest perpendicular component is in B, and the largest is in C. 14. (a) The static frictional force is balancing the component of the blocks weight that points down the slope of the incline. This component is smallest in B and greatest in A. 15. (b) The static frictional force that blocks A and B exert on each other has a magnitude f. The force that B exerts on A is directed to the right (the positive direction), while the force that A exerts on B is directed to the left. Blocks B and C also exert static frictional forces on each other, but these forces have a magnitude 2f, because the normal force pressing B and C together is twice the normal force pressing A and B together. The force that C exerts on B is directed to the right, while the force that B exerts on C is directed to the left. In summary, then, block A experiences a single frictional force +f, which is the net frictional force; block B experiences two frictional forces, and +2f, the net frictional force being +2f = +f; f f block C experiences a single frictional force +2f, which is the net frictional force. It follows that fs, A = fs, B = fs, C/2. 16. (c) The magnitude of the kinetic frictional force is proportional to the magnitude of the normal force. The normal force is smallest in B, because the vertical component of F Chapter 4 Answers to Focus on Concepts Questions 161 compensates for part of the blocks weight. In contrast, the normal force is greatest in C, because the vertical component of F adds to the weight of the block. 17. (d) Acceleration is inversely proportional to mass, according to Newtons second law. This law also indicates that acceleration is directly proportional to the net force. The frictional force is the net force acting on a block, and its magnitude is directly proportional to the magnitude of the normal force. However, in each of the pictures the normal force is directly proportional to the weight and, thus, the mass of a block. The inverse proportionality of the acceleration to mass and the direct proportionality of the net force to mass offset each other. The result is that the deceleration is the same in each case. 18. (e) In B the tension T is the smallest, because three rope segments support the weight W of the block, with the result that 3T = W, or T = W/3. In A the tension is the greatest, because only one rope segment supports the weight of the block, with the result that T = W. 19. (c) Since the engines are shut down and since nothing is nearby to exert a force, the net force acting on the probe is zero, and its acceleration must be zero, according to Newtons second law. With zero acceleration the probe is in equilibrium. 20. (a) The hallmark of an object in equilibrium is that it has no acceleration. Therefore, an object in equilibrium need not be at rest. It can be moving with a constant velocity. 21. (b) Since the object is not in equilibrium, it must be accelerating. Newtons second law, in turn, implies that a net force must be present to cause the acceleration. Whether the net forces arises from a single force, two perpendicular forces, or three forces is not important, because only the net force appears in the second law. 22. (d) The block is at rest and, therefore, in equilibrium. According to Newtons second law, then, the net force acting on the block in a direction parallel to the inclined surface of the incline must be zero. This means that the force of static friction directed up the incline must balance the component of the blocks weight directed down the incline [(8.0 kg) (9.8 m/s2) sin 22 = 29 N]. 23. (b) Since the boxes move at a constant velocity, they have no acceleration and are, therefore, in equilibrium. According to Newtons second law, the net force acting on each box must be zero. Thus, Newtons second law applied to each box gives two equations in two unknowns, the magnitude of the tension in the rope between the boxes and the kinetic frictional force that acts on each box. Note that the frictional forces acting on the boxes are identical, because the boxes are identical. Solving these two equations shows that the tension is one-half of the applied force. 24. 31 kgm/s2 25. 517 N 162 FORCES AND NEWTON'S LAWS OF MOTION CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION PROBLEMS _____________________________________________________________________________ _ 1. REASONING AND SOLUTION According to Newtons second law, the acceleration is a = F/m. Since the pilot and the plane have the same acceleration, we can write F F = m PILOT m PLANE F m PLANE ( F ) PILOT = mPILOT or Therefore, we find 3.7 104 N = 93 N 4 3.1 10 kg _____________________________________________________________________________ _ ( F ) PILOT = ( 78 kg ) 2. REASONING Suppose the bobsled is moving along the + x direction. There are two forces acting on it that are parallel to its motion; a force + Fx propelling it forward and a force of 450 N that is resisting its motion. The net force is the sum of these two forces. According to Newtons second law, Equation 4.2a, the net force is equal to the mass of the bobsled times its acceleration. Since the mass and acceleration are known, we can use the second law to determine the magnitude of the propelling force. SOLUTION a. Newtons second law states that + Fx 450 N = max 1 44 2 4 43 Fx (4.2a) Solving this equation for Fx gives ( ) Fx = max + 450 N = ( 270 kg ) 2.4 m/s 2 + 450 N = 1100 N b. The magnitude of the net force that acts on the bobsled is ( ) Fx = max = ( 270 kg ) 2.4 m/s 2 = 650 N (4.2a) ____________________________________________________________________________________________ Chapter 4 Problems 3. 163 REASONING In each case, we will apply Newtons second law. Remember that it is the net force that appears in the second law. The net force is the vector sum of both forces. SOLUTION a. We will use Newtons second law, Fx = max, to find the force F2. Taking the positive x direction to be to the right, we have F1 + F2 14 2 43 = ma x Fx so F2 = max F1 F2 = (3.0 kg)(+5.0 m/s2) (+9.0 N) = +6 N b. Applying Newtons second law again gives F2 = max F1 = (3.0 kg)(5.0 m/s2) (+9.0 N) = 24 N c. An application of Newtons second law gives F2 = max F1 = (3.0 kg)(0 m/s2) (+9.0 N) = 9.0 N _____________________________________________________________________________ _ 4. REASONING According to Newtons second law, Equation 4.1, the average net force F is equal to the product of the objects mass m and the average acceleration a . The average acceleration is equal to the change in velocity divided by the elapsed time (Equation 2.4), where the change in velocity is the final velocity v minus the initial velocity v0. SOLUTION The average net force exerted on the car and riders is v v0 ( ) 45 m/s 0 m/s = 5.5 103 kg = 3.5 104 N t t0 7.0 s _____________________________________________________________________________ _ F = ma = m 5. SSM REASONING The net force acting on the ball can be calculated using Newton's second law. Before we can use Newton's second law, however, we must use Equation 2.9 from the equations of kinematics to determine the acceleration of the ball. SOLUTION According to Equation 2.9, the acceleration of the ball is given by 164 FORCES AND NEWTON'S LAWS OF MOTION a= 2 v 2 v0 2x Thus, the magnitude of the net force on the ball is given by 2 v 2 v0 (45 m/s) 2 (0 m/s) 2 F = ma = m = (0.058 kg) = 130 N 2x 2(0.44 m) _____________________________________________________________________________ _ 6. REASONING AND SOLUTION The acceleration is obtained from x = v0t + 1 2 at2 where v0 = 0 m/s. So a = 2x/t2 Newtons second law gives 2 ( 18 m ) 2x = 2900 N F = ma = m 2 = ( 72 kg ) ( 0.95 s ) 2 t _____________________________________________________________________________ _ 7. SSM REASONING AND SOLUTION The acceleration required is a= 2 v 2 v0 2x 2 ( 15.0 m/s ) = = 2.25 m/s 2 2 ( 50.0 m ) Newton's second law then gives the magnitude of the net force as F = ma = (1580 kg)(2.25 m/s2) = 3560 N ____________________________________________________________________________________________ 8. REASONING We do not have sufficient information to calculate the average net force applied to the fist by taking the vector sum of the individually applied forces. However, we have the mass m of the fist, as well as its initial velocity (v0 = 0 m/s, since the fist starts from rest), final velocity (v = 8.0 m/s), and the elapsed time (t = 0.15 s). Therefore we can use v v v0 = Equation 2.4 a = to determine the average acceleration a of the fist and then t t use Equation 4.1 (Newtons second law, F = ma ) to find the average net force F applied to the fist. Chapter 4 Problems SOLUTION Inserting the relation a = average net force applied to the fist: 165 v v v0 = into Newtons second law yields the t t v v0 8.0 m/s 0 m/s F = ma = m = ( 0.70 kg ) = 37 N 0.15 s t 9. SSM WWW REASONING Let due east be chosen as the positive direction. Then, when both forces point due east, Newton's second law gives FA + F = ma1 14 2 4B 3 F (1) where a1 = 0.50 m/s 2 . When FA points due east and FB points due west, Newton's second law gives FA F = ma2 14 2 4B 3 F (2) where a2 = 0.40 m/s 2 . These two equations can be used to find the magnitude of each force. SOLUTION a. Adding Equations 1 and 2 gives FA = m ( a1 + a2 ) = 2 ( 8.0 kg ) ( 0.50 m / s 2 + 0.40 m / s2 ) 2 = 3.6 N b. Subtracting Equation 2 from Equation 1 gives FB = m ( a1 a2 ) 2 = ( 8.0 kg ) ( 0.50 m / s2 0.40 m / s 2 ) 2 = 0.40 N ____________________________________________________________________________________________ 166 FORCES AND NEWTON'S LAWS OF MOTION 10. REASONING From Newtons second law, we know that the net force F acting on the electron is directly proportional to its acceleration, so in part a we will first find the electrons acceleration. The problem text gives the electrons initial velocity (v0 = +5.40105 m/s) and final velocity (v = +2.10106 m/s), as well as its displacement (x = +0.038 m) during the interval of acceleration. The elapsed time is not known, so we 2 2 will use Equation 2.9 v = v0 + 2ax to calculate the electrons acceleration. Then we will ( ) find the net force acting on the electron from Equation 4.1 ( F = ma ) and the electrons mass. Because F1 points in the +x direction and F2 points in the x direction, the net force acting on the electron is F = F1 F2 . In part b of the problem, we will rearrange this expression to obtain the magnitude of the second electric force. SOLUTION a. Solving Equation 2.9 for the electrons acceleration, we find that a= 2 v 2 v0 2x ( +2.10 106 m/s ) ( +5.40 105 m/s ) = 2 2 ( +0.038 m ) 2 = +5.42 1013 m/s2 Newtons 2nd law of motion then gives the net force causing the acceleration of the electron: F = ma = ( 9.11 1031 kg ) ( +5.42 1013 m/s 2 ) = +4.94 10 17 N b. The net force acting on the electron is F = F1 F2 , so the magnitude of the second electric force is F2 = F1 F , where F is the net force found in part a: F2 = F1 F = 7.50 1017 N 4.94 1017 N = 2.56 10 17 N 11. REASONING Newtons second law gives the acceleration as a = (F)/m. Since we seek only the horizontal acceleration, it is the x component of this equation that we will use; ax = (Fx)/m. For completeness, however, the free-body diagram will include the vertical forces also (the normal force FN and the weight W). Chapter 4 Problems SOLUTION The free-body diagram is shown at the right, where 167 +y F1 = 59.0 N F2 = 33.0 N FN = 70.0 F2 When F1 is replaced by its x and y components, we obtain the free body diagram in the following drawing. +x F1 W Choosing right to be the positive direction, we have F F cos F2 ax = x = 1 m m ax ( 59.0 N ) cos 70.0 ( 33.0 N ) = 7.00 kg = 1.83 m/s +y FN 2 F1cos F2 The minus sign indicates that the horizontal acceleration points to the left . +x F1sin W 12. REASONING The net force F has a horizontal component Fx and a vertical component Fy. Since these components are perpendicular, the Pythagorean theorem applies (Equation 1.7), and the magnitude of the net force is F = ( Fx ) 2 ( + Fy ) 2 . Newtons second law allows us to express the components of the net force acting on the ball in terms of its mass and the horizontal and vertical components of its acceleration: Fx = ma x , Fy = ma y (Equations 4.2a and 4.2b). SOLUTION Combining the Pythagorean theorem with Newtons second law, we obtain the magnitude of the net force acting on the ball: F = ( Fx ) 2 ( + Fy = ( 0.430 kg ) ) 2 = ( max ) 2 ( + ma y ) 2 ( 810 m/s2 ) + ( 1100 m/s 2 ) 2 2 2 = m ax + a 2 y = 590 N 168 FORCES AND NEWTON'S LAWS OF MOTION 13. SSM REASONING To determine the acceleration we will use Newtons second law F = ma. Two forces act on the rocket, the thrust T and the rockets weight W, which is mg = (4.50 105 kg)(9.80 m/s2) = 4.41 106 N. Both of these forces must be considered when determining the net force F. The direction of the acceleration is the same as the direction of the net force. SOLUTION In constructing the free-body diagram for the rocket we choose upward and to the right as the positive directions. The free-body diagram is as follows: +y T The x component of the net force is Fx = T cos 55.0 = 7.50 106 N cos 55.0 = 4.30 106 N ( Ty ) 55.0 The y component of the net force is W ( Tx = T sin 55.0 +x = T cos 55.0 ) Fy = T sin 55.0 W = 7.50 106 N sin 55.0 4.41 106 N = 1.73 106 N The magnitudes of the net force and of the acceleration are F = a= ( Fx ) 2 ( + Fy ) 2 m ( Fx ) 2 + ( Fy ) 2 ( 4.30 106 N ) + ( 1.73 106 N ) 2 = 2 5 4.50 10 kg = 10.3 m/s 2 The direction of the acceleration is the same as the direction of the net force. Thus, it is directed above the horizontal at an angle of Fy = tan 1 F x 6 1 1.73 10 N = tan 4.30 106 N = 21.9 14. REASONING The acceleration of the sky diver can be obtained directly from Newtons second law as the net force divided by the sky divers mass. The net force is the vector sum of the sky divers weight and the drag force. SOLUTION From Newtons second law, F = ma (Equation 4.1), the sky divers acceleration is a= f F m + W Free-body diagram Chapter 4 Problems 169 The free-body diagram shows the two forces acting on the sky diver, his weight W and the drag force f. The net force is F = f W . Thus, the acceleration can be written as a= f W m The acceleration of the sky diver is a= f W 1027 N 915 N = = +1.20 m/s 2 m 93.4 kg Note that the acceleration is positive, indicating that it points upward . ____________________________________________________________________________________________ 15. REASONING Newtons second law, F = ma, states that a net force of F must act on an object of mass m in order to impart an acceleration a to the object. In the impact shock test the box is subjected to a large deceleration and, hence, a correspondingly large net force. To determine the net force we will determine the deceleration in a kinematics calculation and use it in Newtons second law. SOLUTION According to Newtons second law, the net force is F = ma, where the acceleration can be determined with the aid of Equation 2.4 (v = v0 + at). According to this equation v v0 a= t Substituting this result for the acceleration into the second law gives v v0 F = ma = m t Since the initial velocity (v0 = +220 m/s), final velocity ( v = 0 m/s), and the duration of the collision (t = 6.5 10 3 s) are known, we find v v0 0 m/s 220 m/s 6 F = m = ( 41 kg ) = 1.39 10 N 3 t 6.5 10 s The minus sign indicates that the net force points opposite to the direction in which the box is thrown, which has been assumed to be the positive direction. The magnitude of the net force is 1.39 106 N , which is over three hundred thousand pounds. 170 FORCES AND NEWTON'S LAWS OF MOTION 16. REASONING Since there is only one force acting on the man in the horizontal direction, it is the net force. According to Newtons second law, Equation 4.1, the man must accelerate under the action of this force. The factors that determine this acceleration are (1) the magnitude and (2) the direction of the force exerted on the man, and (3) the mass of the man. When the woman exerts a force on the man, the man exerts a force of equal magnitude, but opposite direction, on the woman (Newtons third law). It is the only force acting on the woman in the horizontal direction, so, as is the case with the man, she must accelerate. The factors that determine her acceleration are (1) the magnitude and (2) the direction of the force exerted on her, and (3) the her mass. SOLUTION a. The acceleration of the man is, according to Equation 4.1, equal to the net force acting on him divided by his mass. aman = F 45 N = = 0.55 m / s 2 (due east) m 82 kg b. The acceleration of the woman is equal to the net force acting on her divided by her mass. awoman = F 45 N = = 0.94 m / s 2 (due west) m 48 kg ______________________________________________________________________________________ 17. REASONING Equations 3.5a ( x = v0 xt + 1 axt 2 ) 2 and 3.5b ( y = v0 yt + 12 a yt 2 ) give the displacements of an object under the influence of constant accelerations ax and ay. We can add these displacements as vectors to find the magnitude and direction of the resultant displacement. To use Equations 3.5a and 3.5b, however, we must have values for ax and ay. We can obtain these values from Newtons second law, provided that we combine the given forces to calculate the x and y components of the net force acting on the duck, and it is here that our solution begins. SOLUTION Let the directions due east and due north, respectively, be the +x and +y directions. Then, the components of the net force are Fx = 0.10 N + ( 0.20 N ) cos 52 = 0.2231 N Fy = ( 0.20 N ) sin 52 = 0.1576 N According to Newtons second law, the components of the acceleration are Chapter 4 Problems ax = ay = Fx m Fy m = 0.2231 N = 0.08924 m/s 2 2.5 kg = 171 0.1576 N = 0.06304 m/s 2 2.5 kg From Equations 3.5a and 3.5b, we now obtain the displacements in the x and y directions: ( ) x = v0 xt + 1 axt 2 = ( 0.11 m/s ) ( 3.0 s ) + 1 0.08924 m/s 2 ( 3.0 s ) = 0.7316 m 2 2 ( 2 ) y = v0 y t + 1 a y t 2 = ( 0 m/s ) ( 3.0 s ) + 1 0.06304 m/s 2 ( 3.0 s ) = 0.2837 m 2 2 2 The magnitude of the resultant displacement is r = x2 + y2 = ( 0.7316 m ) 2 + ( 0.2837 m ) 2 = 0.78 m The direction of the resultant displacement is 0.2837 m = tan 1 = 21 south of east 0.7316 m ____________________________________________________________________________________________ ( ) 2 18. REASONING For both the tug and the asteroid, Equation 2.8 x = v0t + 1 at applies with 2 v0 = 0 m/s, since both are initially at rest. In applying this equation, we must be careful and use the proper acceleration for each object. Newtons second law indicates that the acceleration is given by a = F/m. In this expression, we note that the magnitudes of the net forces acting on the tug and the asteroid are the same, according to Newtons action-reaction law. The masses of the tug and the asteroid are different, however. Thus, the distance traveled for either object is given by, where we use for F only the magnitude of the pulling force F 2 x = v0t + 1 at 2 = 1 t 2 2 m SOLUTION Let L be the initial distance between the tug and the asteroid. When the two objects meet, the distances that each has traveled must add up to equal L. Therefore, L = xT + x A = 1 aT t 2 + 1 a At 2 2 2 F 2 1 F 2 1 1 12 L= 1 + t + 2 t = 2 F t 2m m m T A T mA 172 FORCES AND NEWTON'S LAWS OF MOTION Solving for the time t gives t= 2L 1 1 F m +m T A = 2 ( 450 m ) 1 1 + ( 490 N ) 3500 kg 6200 kg = 64 s ____________________________________________________________________________________________ 19. SSM WWW REASONING We first determine the acceleration of the boat. Then, using Newton's second law, we can find the net force F that acts on the boat. Since two of the three forces are known, we can solve for the unknown force FW once the net force F is known. SOLUTION Let the direction due east be the positive x direction and the direction due north be the positive y direction. The x and y components of the initial velocity of the boat are then v0 x = (2.00 m/s) cos 15.0 = 1.93 m/s v0 y = (2.00 m/s) sin 15.0 = 0.518 m/s Thirty seconds later, the x and y velocity components of the boat are vx = (4.00 m/s) cos 35.0 = 3.28 m/s v y = (4.00 m/s) sin 35.0 = 2.29 m/s Therefore, according to Equations 3.3a and 3.3b, the x and y components of the acceleration of the boat are v v 3.28 m/s 1.93 m/s ax = x 0 x = = 4.50 102 m/s 2 t 30.0 s ay = v y v0 y t = 2.29 m/s 0.518 m/s = 5.91 102 m/s 2 30.0 s Thus, the x and y components of the net force that act on the boat are Fx = ma x = (325 kg) (4.50 10 2 m/s 2 ) = 14.6 N Fy = ma y = (325 kg) (5.9110 2 m/s 2 ) = 19.2 N Chapter 4 Problems 173 The following table gives the x and y components of the net force F and the two known forces that act on the boat. The fourth row of that table gives the components of the unknown force FW . Force x-Component y-Component F 14.6 N 19.2 N F1 (31.0 N) cos 15.0 = 29.9 N (31.0 N) sin 15.0 = 8.02 N F2 (23.0 N ) cos 15.0 = 22.2 N (23.0 N) sin 15.0 = 5.95 N FW = F F1 F2 14.6 N 29.9 N + 22.2 N = 6.9 N 19.2 N 8.02 N + 5.95 N = 17.1 N The magnitude of FW is given by the Pythagorean theorem as FW = (6.9 N) 2 + (17.1N ) 2 = 18.4 N The angle that FW makes with the x axis is 17.1 N = tan 1 = 68 6.9 N 17.1 N 6.9 N Therefore, the direction of FW is 68, north of east . ____________________________________________________________________________________________ 20. REASONING The gravitational force acting on each object is specified by Newtons law of universal gravitation. The acceleration of each object when released can be determined with the aid of Newtons second law. We recognize that the gravitational force is the only force acting on either object, so that it is the net force to use when applying the second law. SOLUTION a. The magnitude of the gravitational force exerted on the rock by the earth is given by Equation 4.3 as Gmearth mrock Frock = = ( 2 rearth 6.67 1011 N m 2 / kg 2 5.98 1024 kg ( 5.0 kg ) )( ( 6.38 10 m ) 6 ) 2 = 49 N 174 FORCES AND NEWTON'S LAWS OF MOTION The magnitude of the gravitational force exerted on the pebble by the earth is Gmearth mpebble Fpebble = = ( 2 rearth 6.67 1011 N m 2 / kg 2 5.98 1024 kg 3.0 10 4 kg )( ( 6.38 106 m ) )( 2 )= 2.9 10 3 N b. According to the second law, the magnitude of the acceleration of the rock is equal to the gravitational force exerted on the rock divided by its mass. arock = Frock mrock Gmearth = = ( 2 rearth 6.67 1011 N m 2 / kg 2 5.98 1024 kg )( ( 6.38 10 m ) 6 2 )= 9.80 m /s 2 According to the second law, the magnitude of the acceleration of the pebble is equal to the gravitational force exerted on the pebble divided by its mass. apebble = Fpebble mpebble = Gmearth 2 rearth ( 6.67 1011 N m2 / kg 2 ) ( 5.98 1024 kg ) = = 2 ( 6.38 106 m ) 9.80 m /s 2 _____________________________________________________________________________ _ 21. REASONING AND SOLUTION a. According to Equation 4.5, the weight of the space traveler of mass m = 115 kg on earth is W = mg = (115 kg) (9.80 m/s 2 ) = 1.13 103 N b. In interplanetary space where there are no nearby planetary objects, the gravitational force exerted on the space traveler is zero and g = 0 m/s2. Therefore, the weight is W = 0 N . Since the mass of an object is an intrinsic property of the object and is independent of its location in the universe, the mass of the space traveler is still m = 115 kg . Chapter 4 Problems 175 B + mg Free-body diagram of jumper ____________________________________________________________________________________________ 22. REASONING With air resistance neglected, only two forces act on the bungee jumper at this instant (see the free-body diagram): the bungee cord pulls up on her with a force B, and the earth pulls down on her with a gravitational force mg. Because we know the jumpers mass and acceleration, we can apply Newtons second law to this free-body diagram and solve for B. SOLUTION We will take the direction of the jumpers acceleration (downward) as negative. Then, the net force acting on the bungee jumper is F = B mg. With Newtons second law (F = ma), this becomes ma = B mg. We now solve for B: ( ) B = ma + mg = m ( a + g ) = ( 55 kg ) 7.6 m/s 2 + 9.80 m/s 2 = 120 N As indicated in the free-body diagram, the direction of the force applied by the bungee cord is upward . 23. REASONING The earth exerts a gravitational force on the raindrop, and simultaneously the raindrop exerts a gravitational force on the earth. This gravitational force is equal in magnitude to the gravitational force that the earth exerts on the raindrop. The forces that the raindrop and the earth exert on each other are Newtons third law (actionreaction) forces. Newtons law of universal gravitation specifies the magnitude of both forces. SOLUTION a. The magnitude of the gravitational force exerted on the raindrop by the earth is given by Equation 4.3: Gmearth mraindrop Fraindrop = = ( 2 rearth 6.67 1011 N m 2 / kg 2 5.98 10 24 kg 5.2 10 7 kg )( ( 6.38 10 m ) 6 )( 2 )= 5.1 106 N 176 FORCES AND NEWTON'S LAWS OF MOTION b. The magnitude of the gravitational force exerted on the earth by the raindrop is Gmearth mraindrop Fearth = = ( 2 rearth 6.67 10 11 N m 2 / kg 2 5.98 1024 kg 5.2 10 7 kg )( ( 6.38 106 m ) )( 2 )= 5.1 106 N _____________________________________________________________________________ _ 24. REASONING Newtons law of universal gravitation indicates that the gravitational force that each uniform sphere exerts on the other has a magnitude that is inversely proportional to the square of the distance between the centers of the spheres. Therefore, the maximum gravitational force between two uniform spheres occurs when the centers of the spheres are as close together as possible, and this occurs when the surfaces of the spheres are touching. Then, the distance between the centers of the spheres is the sum of the two radii. SOLUTION When the bowling ball and the billiard ball are touching, the distance between their centers is r = rBowling + rBilliard. Using this expression in Newtons law of universal gravitation gives F= GmBowling mBilliard GmBowling mBilliard = ( rBowling + rBilliard ) ( 6.67 1011 N m 2 / kg 2 ) ( 7.2 kg ) ( 0.38 kg ) = 9.6 109 N = r2 2 ( 0.11 m + 0.028 m ) 2 25. SSM REASONING AND SOLUTION a. Combining Equations 4.4 and 4.5, we see that the acceleration due to gravity on the surface of Saturn can be calculated as follows: gSaturn = G M Saturn 2 rSaturn ( = 6.67 10 11 2 N m /kg 2 ( 5.67 1026 kg ) = ) (6.00 107 m)2 10.5 m/s 2 b. The ratio of the persons weight on Saturn to that on earth is WSaturn Wearth = mgSaturn mgearth = gSaturn gearth = 10.5 m/s 2 = 1.07 9.80 m/s 2 ____________________________________________________________________________________________ Chapter 4 Problems 177 26. REASONING As discussed in Conceptual Example 7, the same net force is required on the moon as on the earth. This net force is given by Newtons second law as F = ma, where the mass m is the same in both places. Thus, from the given mass and acceleration, we can calculate the net force. On the moon, the net force comes about due to the drive force and the opposing frictional force. Since the drive force is given, we can find the frictional force. SOLUTION Newtons second law, with the direction of motion taken as positive, gives F = ma ( 1430 N ) or ( )( f = 5.90 103 kg 0.220 m/s 2 ) Solving for the frictional force f , we find ( )( ) f = ( 1430 N ) 5.90 103 kg 0.220 m/s 2 = 130N ____________________________________________________________________________________________ 27. SSM REASONING AND SOLUTION According to Equations 4.4 and 4.5, the weight of an object of mass m at a distance r from the center of the earth is mg = GM E m r2 In a circular orbit that is 3.59 107 m above the surface of the earth ( radius = 6.38 106 m , mass = 5.98 1024 kg ), the total distance from the center of the earth is r = 3.59 107 m + 6.38 106 m . Thus the acceleration g due to gravity is g= GM E r 2 = (6.67 1011N m 2 /kg 2 )(5.98 1024 kg) = (3.59 107 m + 6.38 106 m) 2 0.223 m/s 2 ____________________________________________________________________________________________ 28. REASONING AND SOLUTION The magnitude of the net force acting on the moon is found by the Pythagorean theorem to be F SM Sun F= 2 FSM F Moon F EM 2 + FEM Earth Newton's law of gravitation applied to the sunmoon (the units have been suppressed) 178 FORCES AND NEWTON'S LAWS OF MOTION FSM = GmS mM 2 rSM = ( 6.67 1011 ) ( 1.99 1030 ) ( 7.35 1022 ) ( 1.50 1011 ) 2 = 4.34 1020 N A similar application to the earth-moon gives FEM = GmE mM 2 rEM = ( 6.67 1011 ) ( 5.98 1024 ) ( 7.35 1022 ) ( 3.85 108 ) 2 = 1.98 10 20 N The net force on the moon is then F= ( 4.34 1020 N ) + ( 1.98 1020 N ) 2 2 = 4.77 10 20 N ____________________________________________________________________________________________ 29. REASONING The magnitude of the gravitational force exerted on the satellite by the earth is given by Equation 4.3 as F = Gmsatellite mearth / r 2 , where r is the distance between the satellite and the center of the earth. This expression also gives the magnitude of the gravitational force exerted on the earth by the satellite. According to Newtons second law, the magnitude of the earths acceleration is equal to the magnitude of the gravitational force exerted on it divided by its mass. Similarly, the magnitude of the satellites acceleration is equal to the magnitude of the gravitational force exerted on it divided by its mass. SOLUTION a. The magnitude of the gravitational force exerted on the satellite when it is a distance of two earth radii from the center of the earth is F= Gmsatellite mearth r2 = ( 6.67 1011 N m2 / kg 2 ) ( 425 kg ) ( 5.98 1024 kg ) 2 ( 2 ) ( 6.38 106 m ) = 1.04 103 N b. The magnitude of the gravitational force exerted on the earth when it is a distance of two earth radii from the center of the satellite is F= Gmsatellite mearth r2 = ( 6.67 1011 N m2 / kg 2 ) ( 425 kg ) ( 5.98 1024 kg ) 2 ( 2 ) ( 6.38 106 m ) = 1.04 103 N c. The acceleration of the satellite can be obtained from Newtons second law. asatellite = F msatellite = 1.04 103 N = 425 kg 2.45 m / s 2 Chapter 4 Problems 179 d. The acceleration of the earth can also be obtained from Newtons second law. aearth = F mearth 1.04 103 N = 1.74 1022 m / s 2 24 5.98 10 kg = ____________________________________________________________________________________________ 30. REASONING The weight of a person on the earth is the gravitational force Fearth that it exerts on the person. The magnitude of this force is given by Equation 4.3 as Fearth = G mearth mperson 2 rearth where rearth is the distance from the center of the earth to the person. In a similar fashion, the weight of the person on another planet is Fplanet = G mplanet mperson 2 rplanet We will use these two expressions to obtain the weight of the traveler on the planet. SOLUTION Dividing Fplanet by Fearth we have Fplanet Fearth G = G mplanet mperson 2 rplanet mearth mperson 2 rearth mplanet = m earth rearth r planet 2 or Fplanet Since we are given that mplanet mearth mplanet = Fearth m earth = 3 and rearth rplanet = rearth r planet 2 1 , the weight of the space traveler on the 2 planet is 2 Fplanet 1 = ( 540.0 N ) ( 3) = 405.0 N 2 180 FORCES AND NEWTON'S LAWS OF MOTION _____________________________________________________________________________ _ 31. SSM REASONING According to Equation 4.4, the weights of an object of mass m on the surfaces of planet A (mass = MA, radius = R ) and planet B (mass = MB , radius = R ) are WA = GM A m and R2 WB = GM Bm R2 The difference between these weights is given in the problem. SOLUTION The difference in weights is WA WB = GM A m R 2 GM Bm R 2 = Gm ( MA MB ) R2 Rearranging this result, we find MA MB ( WA WB ) R 2 = = Gm ( 3620 N ) ( 1.33 107 m ) (6.67 10 11N m 2 /kg 2 ) ( 5450 kg ) 2 = 1.76 10 24 kg ____________________________________________________________________________________________ 32. REASONING AND SOLUTION The figure at the right shows the three spheres with sphere 3 being the sphere of unknown mass. Sphere 3 feels a force F31 due to the presence of sphere 1, and a force F32 due to the presence of sphere 2. The net force on sphere 3 is the resultant of F31 and F32. Note that since the spheres form an equilateral triangle, each interior angle is 60. Therefore, both F31 and F32 make a 30 angle with the vertical line as shown. 3 F F 31 32 1.20 m 30 1 2 Furthermore, F31 and F32 have the same magnitude given by GMm3 F= r2 where M is the mass of either sphere 1 or 2 and m3 is the mass of sphere 3. The components of the two forces are shown in the following drawings: Chapter 4 Problems F 31 F cos 30.0 F cos F sin 181 F 30.0 32 F sin Clearly, the horizontal components of the two forces add to zero. Thus, the net force on sphere 3 is the resultant of the vertical components of F31 and F32: F3 = 2 F cos = 2 GMm3 r2 cos The acceleration of sphere 3 is given by Newton's second law: (6.67 10 GM a3 = = 2 2 cos = 2 m3 r F3 11 N m 2 /kg 2 ) ( 2.80 kg ) ( 1.20 m ) 2 cos 30.0 = 2.25 10 10 m/s 2 ____________________________________________________________________________________________ 33. SSM WWW REASONING AND SOLUTION There are two forces that act on the balloon; they are, the combined weight of the balloon and its load, Mg, and the upward buoyant force FB . If we take upward as the positive direction, then, initially when the balloon is motionless, Newton's second law gives FB Mg = 0 . If an amount of mass m is dropped overboard so that the balloon has an upward acceleration, Newton's second law for this situation is FB ( M m) g = ( M m) a But FB = mg , so that Mg ( M m ) g = mg = ( M m ) a Solving for the mass m that should be dropped overboard, we obtain Ma (310 kg )(0.15 m/s 2 ) = = 4.7 kg g + a 9.80 m/s 2 + 0.15 m/s 2 _____________________________________________________________________________ _ m= 34. REASONING AND SOLUTION acceleration, it follows that Since both motions are characterized by constant 182 FORCES AND NEWTON'S LAWS OF MOTION yJ yE = 1 a t2 2 JJ 1 a t2 2 EE where the subscripts designate those quantities that pertain to Jupiter and Earth. Since both objects fall the same distance (yJ = yE), the above ratio is equal to unity. Solving for the ratio of the times yields tJ tE = aE aJ = 2 GM E / RE 2 GM J / RJ = RJ ME RE MJ = ( 11.2 ) 1 = 0.628 318 ____________________________________________________________________________________________ 35. REASONING The gravitational force that the sun exerts on a person standing on the earth 2 is given by Equation 4.3 as Fsun = GM sun m / rsun-earth , where Msun is the mass of the sun, m is the mass of the person, and rsun-earth is the distance from the sun to the earth. Likewise, the gravitational force that the moon exerts on a person standing on the earth is given by 2 Fmoon = GM moon m / rmoon-earth , where Mmoon is the mass of the moon and rmoon-earth is the distance from the moon to the earth. These relations will allow us to determine whether the sun or the moon exerts the greater gravitational force on the person. SOLUTION Taking the ratio of Fsun to Fmoon, and using the mass and distance data from the inside of the texts front cover, we find GM sun m Fsun Fmoon 2 M rsun-earth = = sun M GM moon m moon 2 rmoon-earth rmoon-earth r sun-earth 2 2 1.99 1030 kg 3.85 108 m = 7.35 1022 kg 1.50 1011 m = 178 Therefore, the sun exerts the greater gravitational force. ____________________________________________________________________________________________ Chapter 4 Problems 183 36. REASONING The gravitational attraction between the planet and the moon is governed by Newtons law of gravitation F = GMm/r2 (Equation 4.3), where M is the planets mass and m is the moons mass. Because the magnitude of this attractive force varies inversely with the square of the distance r between the center of the moon and the center of the planet, the maximum force Fmax occurs at the minimum distance rmin, and the minimum force Fmin at the maximum distance rmax. The problem text states that Fmax exceeds Fmin by 11%, or Fmax = 1.11 Fmin. This expression can be rearranged to give the ratio of the forces: Fmax/ Fmin = 1.11. We will use Equation 4.3 to compute the desired distance ratio in terms of this force ratio. SOLUTION From Equation 4.3, the ratio of the maximum gravitational force to the minimum gravitational force is Fmax Fmin = GMm 2 rmin GMm 2 rmax 1 = 2 2 rmin rmax =2 1 rmin 2 rmax Taking the square root of both sides of this expression and substituting the ratio of forces Fmax/ Fmin = 1.11 yields the ratio of distances: rmax rmin = Fmax Fmin = 1.11 = 1.05 The moons maximum distance from the center of the planet is therefore about 5% larger than its minimum distance. 37. REASONING We place the third particle (mass = m3) as shown in the following drawing: L D m3 m 2m The magnitude of the gravitational force that one particle exerts on another is given by Newtons law of gravitation as F = Gm1m2/r2. Before the third particle is in place, this law indicates that the force on each particle has a magnitude Fbefore = Gm2m/L2. After the third particle is in place, each of the first two particles experiences a greater net force, because the third particle also exerts a gravitational force on them. SOLUTION For the particle of mass m, we have 184 FORCES AND NEWTON'S LAWS OF MOTION Fafter = Fbefore Gmm3 Gm 2m 2 D L2 = L m3 + 1 Gm2m 2mD 2 L2 2 + For the particle of mass 2m, we have G 2mm3 Fafter Fbefore = ( L D) 2 + Gm2m L2 Gm2m L2 = L2 m3 m( L D) 2 +1 Since Fafter/Fbefore = 2 for both particles, we have L2 m3 2mD 2 +1 = L2 m3 m ( L D) 2 +1 or 2D2 = ( L D ) 2 Expanding and rearranging this result gives D 2 + 2 LD L2 = 0 , which can be solved for D using the quadratic formula: D= 2 L ( 2 L ) 2 4 ( 1) ( L2 ) 2 ( 1) = 0.414 L or 2.414 L The negative solution is discarded because the third particle lies on the + x axis between m and 2m. Thus, D = 0.414 L . ____________________________________________________________________________________________ 38. REASONING In each case the object is in equilibrium. According to Equation 4.9b, Fy = 0, the net force acting in the y (vertical) direction must be zero. The net force is composed of the weight of the object(s) and the normal force exerted on them. SOLUTION a. There are three vertical forces acting on the crate: an upward normal force + FN that the floor exerts, the weight m1g of the crate, and the weight m2g of the person standing on the crate. Since the weights act downward, they are assigned negative numbers. Setting the sum of these forces equal to zero gives Chapter 4 Problems 185 F + ( m g ) + ( m2 g ) = 0 1 N 4 4 4 12 4 4 4 4 4 4 4 3 Fy The magnitude of the normal force is FN = m1g + m2g = (35 kg + 65 kg)(9.80 m/s2) = 980 N b. There are only two vertical forces acting on the person: an upward normal force + FN that the crate exerts and the weight m2g of the person. Setting the sum of these forces equal to zero gives F + ( m2 g ) = 0 1 N 4 2 4 43 4 Fy The magnitude of the normal force is FN = m2g = (65 kg)(9.80 m/s2) = 640 N ____________________________________________________________________________________________ 39. SSM REASONING The book is kept from falling as long as the total static frictional force balances the weight of the book. The forces that act on the book are shown in the following free-body diagram, where P is the pressing force applied by each hand. fsMAX fsMAX P P W In this diagram, note that there are two pressing forces, one from each hand. Each hand also applies a static frictional force, and, therefore, two static frictional forces are shown. The maximum static frictional force is related in the usual way to a normal force FN, but in this problem the normal force is provided by the pressing force, so that FN = P. SOLUTION Since the frictional forces balance the weight, we have 2 f sMAX = 2 ( s FN ) = 2 ( s P ) = W Solving for P, we find that 186 FORCES AND NEWTON'S LAWS OF MOTION P= W 31 N = = 39 N 2 s 2 ( 0.40 ) ____________________________________________________________________________________________ 40. REASONING AND SOLUTION a. The apparent weight of the person is given by Equation 4.6 as F N = mg + ma = (95.0 kg)(9.80 m/s2 + 1.80 m/s2) = 1.10 103 N b. FN = (95.0 kg)(9.80 m/s2) = 931 N c. FN = (95.0 kg)(9.80 m/s2 1.30 m/s2) = 808 N ____________________________________________________________________________________________ 41. REASONING As the drawing shows, the normal FN force FN points perpendicular to the hill, while the weight W points vertically down. Since the car does not leave the surface of the hill, there is no acceleration in this perpendicular direction. Therefore, the magnitude of the perpendicular component of the weight W cos must equal W cos the magnitude of the normal force, FN = W cos . Thus, the W magnitude of the normal force is less than the magnitude of the weight. As the hill becomes steeper, increases, and cos decreases. Consequently, the normal force decreases as the hill becomes steeper. The magnitude of the normal force does not depend on whether the car is traveling up or down the hill. SOLUTION a. From the REASONING, we have that FN = W cos . The ratio of the magnitude of the normal force to the magnitude W of the weight is FN W = W cos = cos 15 = 0.97 W b. When the angle is 35, the ratio is W cos = cos 35 = 0.82 W W _____________________________________________________________________________ _ FN = Chapter 4 Problems 187 42. REASONING The reading on the bathroom scale is proportional to the normal force it exerts on the man. When he simply stands on the scale, his acceleration FN1 is zero, so the normal force pushing up on him balances the downward pull of gravity: FN1 = mg (see P FN2 the free-body diagram). Thus, the first reading on the scale is his actual mass m, the ratio of the normal force the scale exerts on him to the acceleration due to gravity: First reading = m = FN1/g = 92.6 kg. With the mg mg chin-up bar helping to support him, the normal force exerted on him by the scale decreases, and the second Standing Pulli reading is the ratio of the reduced normal force FN2 to the acceleration due to gravity: Second reading = FN2/g = 75.1 kg. Lastly, we note that the magnitude P of the force the chin-up bar exerts on the man is exactly equal to the magnitude P of the force that the man exerts on the chin-up bar. This prediction is due to Newtons third law. Therefore, it is a value for P that we seek. SOLUTION When the man is pulling down on the chin-up bar, there are two upward forces acting on him (see the second part of the drawing), and he is still at rest, so the sum of these two forces balances the downward pull of gravity: FN2 + P = mg, or P = mg FN 2 . Since the second reading on the scale is equal to FN2/g, the normal force the scale exerts on him is FN2 = (Second reading)g. Thus we obtain the magnitude P of the force the man exerts on the chin-up bar: P = mg FN2 = mg (Second reading) g = ( m Second reading ) g ( ) = ( 92.6 kg 75.1 kg ) 9.80 m/s 2 = 172 N 43. REASONING As shown in the free-body diagram below, three forces act on the car: the static frictional force fs (directed up the hill), the normal force FN (directed perpendicular to the road), and its weight mg. As it sits on the hill, the car has an acceleration of zero (ax = ay = 0 m/s2). Therefore, the net force acting on the car in the x direction must be zero ( Fx = 0 ) and the net force in the y direction must be zero ( Fy = 0 ) . These two relations will allow us to find the normal force and the static frictional force. SOLUTION a. Applying Newtons second ( Fy = 0 ) yields law to the y direction Fy = + FN mg cos15 = 0 (4.2b) 188 FORCES AND NEWTON'S LAWS OF MOTION where the term mg cos 15 is the y component of the cars weight (negative, because this component points along the negative y axis). Solving for the magnitude FN of the normal force, we obtain FN = mg cos15 = ( 1700 kg ) ( 9.80 m/s 2 ) cos15 = 1.6 104 N b. Applying Newtons second law to the x direction ( Fx = 0 ) gives Fx = + mg sin15 fs = 0 (4.2a) where the term mg sin 15 is the x component of the cars weight. Solving this expression for the static frictional force gives fs = mg sin15 = ( 1700 kg ) ( 9.80 m/s 2 ) sin15 = 4.3 103 N 44. REASONING a. Since the refrigerator does not move, the static frictional force must be equal in magnitude, but opposite in direction, to the horizontal pushing force that the person exerts on the refrigerator. b. The magnitude of the maximum static frictional force is given by Equation 4.7 as fsMAX = s FN . This is also the largest possible force that the person can exert on the refrigerator before it begins to move. Thus, the factors that determine this force magnitude are the coefficient of static friction s and the magnitude FN of the normal force (which is equal to the weight of the refrigerator in this case). SOLUTION a. Since the refrigerator does not move, it is in equilibrium, and the magnitude of the static frictional force must be equal to the magnitude of the horizontal pushing force. Thus, the magnitude of the static frictional force is 267 N . The direction of this force must be opposite to that of the pushing force, so the static frictional force is in the +x direction . b. The magnitude of the largest pushing force is given by Equation 4.7 as fsMAX = s FN = s mg = (0.65)(57 kg)(9.80 m/s 2 ) = 360 N ____________________________________________________________________________________________ 45. SSM REASONING AND SOLUTION Four forces act on the sled. They are the pulling force P, the force of kinetic friction fk , the weight mg of the sled, and the normal force FN Chapter 4 Problems 189 exerted on the sled by the surface on which it slides. The following figures show free-body diagrams for the sled. In the diagram on the right, the forces have been resolved into their x and y components. y P sin y FN P fk FN fk P cos x mg x mg Since the sled is pulled at constant velocity, its acceleration is zero, and Newton's second law in the direction of motion is (with right chosen as the positive direction) Fx = P cos f k = ma x = 0 From Equation 4.8, we know that f k = k FN , so that the above expression becomes P cos k FN = 0 (1) In the vertical direction, Fy = P sin + FN mg = ma y = 0 (2) Solving Equation (2) for the normal force, and substituting into Equation (1), we obtain P cos k ( mg P sin ) = 0 Solving for k , the coefficient of kinetic friction, we find k = P cos (80.0 N) cos 30.0 = = 0.444 mg P sin (20.0 kg) (9.80 m/s 2 ) (80.0 N) sin 30.0 ____________________________________________________________________________________________ 46. REASONING In each of the three cases under consideration the kinetic frictional force is given by fk = kFN. However, the normal force FN varies from case to case. To determine the normal force, we use Equation 4.6 ( FN = mg + ma) and thereby take into account the 190 FORCES AND NEWTON'S LAWS OF MOTION acceleration of the elevator. The normal force is greatest when the elevator accelerates upward (a positive) and smallest when the elevator accelerates downward (a negative). SOLUTION a. When the elevator is stationary, its acceleration is a = 0 m/s2. Using Equation 4.6, we can express the kinetic frictional force as f k = k FN = k ( mg + ma ) = k m ( g + a ) ( )( ) = ( 0.360 ) ( 6.00 kg ) 9.80 m/s 2 + 0 m/s 2 = 21.2 N b. When the elevator accelerates upward, a = +1.20 m/s2. Then, f k = k FN = k ( mg + ma ) = k m ( g + a ) ( )( ) = ( 0.360 ) ( 6.00 kg ) 9.80 m/s 2 + 1.20 m/s 2 = 23.8 N c. When the elevator accelerates downward, a = 1.20 m/s2. Then, f k = k FN = k ( mg + ma ) = k m ( g + a ) ( )( ) = ( 0.360 ) ( 6.00 kg ) 9.80 m/s 2 + 1.20 m/s 2 = 18.6 N ____________________________________________________________________________________________ 47. REASONING The magnitude of the kinetic frictional force is given by Equation 4.8 as the coefficient of kinetic friction times the magnitude of the normal force. Since the slide into second base is horizontal, the normal force is vertical. It can be evaluated by noting that there is no acceleration in the vertical direction and, therefore, the normal force must balance the weight. To find the players initial velocity v0, we will use kinematics. The time interval for the slide into second base is given as t = 1.6 s. Since the player comes to rest at the end of the slide, his final velocity is v = 0 m/s. The players acceleration a can be obtained from Newtons second law, since the net force is the kinetic frictional force, which is known from part (a), and the mass is given. Since t, v, and a are known and we seek v0, the appropriate kinematics equation is Equation 2.4 (v = v0 + at). SOLUTION a. Since the normal force FN balances the weight mg, we know that FN = mg. Using this fact and Equation 4.8, we find that the magnitude of the kinetic frictional force is ( ) f k = k FN = k mg = ( 0.49 ) ( 81 kg ) 9.8 m/s 2 = 390 N Chapter 4 Problems 191 b. Solving Equation 2.4 ( v = v0 + at) for v0 gives v0 = v at. Taking the direction of the players slide to be the positive direction, we use Newtons second law and Equation 4.8 for the kinetic frictional force to write the acceleration a as follows: a= F k mg = = k g m m The acceleration is negative, because it points opposite to the players velocity, since the player slows down during the slide. Thus, we find for the initial velocity that ( ) v0 = v ( k g ) t = 0 m/s ( 0.49 ) 9.8 m/s 2 ( 1.6 s ) = +7.7 m/s 48. REASONING AND SOLUTION The deceleration produced by the frictional force is a= fk m = k mg m = k g The speed of the automobile after 1.30 s have elapsed is given by Equation 2.4 as v = v0 + at = v0 + ( k g ) t = 16.1 m/s ( 0.720 ) ( 9.80 m/s 2 ) ( 1.30 s ) = 6.9 m/s ____________________________________________________________________________________________ 49. SSM REASONING Let us assume that the skater is moving horizontally along the + x axis. The time t it takes for the skater to reduce her velocity to vx = +2.8 m/s from v0x = +6.3 m/s can be obtained from one of the equations of kinematics: vx = v0 x + a x t (3.3a) The initial and final velocities are known, but the acceleration is not. We can obtain the acceleration from Newtons second law ( Fx = max , Equation 4.2a ) in the following manner. The kinetic frictional force is the only horizontal force that acts on the skater, and, since it is a resistive force, it acts opposite to the direction of the motion. Thus, the net force in the x direction is Fx = f k , where fk is the magnitude of the kinetic frictional force. Therefore, the acceleration of the skater is a x = Fx /m = f k / m . The magnitude of the frictional force is f k = k FN (Equation 4.8), where k is the coefficient of kinetic friction between the ice and the skate blades and FN is the magnitude of the normal force. There are two vertical forces acting on the skater: the upward-acting 192 FORCES AND NEWTON'S LAWS OF MOTION normal force FN and the downward pull of gravity (her weight) mg. Since the skater has no vertical acceleration, Newton's second law in the vertical direction gives (taking upward as the positive direction) Fy = FN mg = 0 . Therefore, the magnitude of the normal force is FN = mg and the magnitude of the acceleration is ax = f k k FN k m g = = = k g m m m SOLUTION Solving the equation v x = v0 x + a x t for the time and substituting the expression above for the acceleration yields t= vx v0 x ax = vx v0 x k g = 2.8 m/s 6.3 m/s = 4.4 s ( 0.081) ( 9.80 m/s 2 ) ____________________________________________________________________________________________ 50. REASONING We assume the car accelerates in the +x direction. The air resistance force fA opposes the cars motion (see the free-body diagram). The frictional force is static, because the tires do not slip, and points in the direction of the cars acceleration. The reason for this is that without friction the cars wheels would simply spin in place, and the cars acceleration would be severely limited. The frictional force has its maximum value fsMAX because we seek the maximum acceleration before slipping occurs. Applying Newtons second law (Fx = max, Equation 4.2a) to the horizontal motion gives fsMAX fsMAX f A = ma x (1) where ax is the maximum acceleration we seek. The air resistance force fA is given, and we ( ) MAX = FN . Because will find the maximum static frictional force from Equation 4.7 fs the cars acceleration has no vertical component, the net vertical force acting on the car must be zero, so the upward normal force FN must balance the two downward forces, the cars weight W and the downforce D: FN = W + D = mg + D (2) Chapter 4 Problems SOLUTION According to Equation 4.7 193 ( fsMAX = s FN ) and Equation (2), the maximum static frictional force the track can exert on the car is fsMAX = FN = ( mg + D ) (3) Now solving Equation (1) for the cars acceleration ax and then substituting Equation (3) for the static frictional force fsMAX , we obtain ax = = fsMAX f A m = s ( mg + D ) f A m ( 0.87 ) ( 690 kg ) ( 9.80 m/s 2 ) + 4060 N 1190 N 690 kg = 12 m/s 2 51. REASONING The diagram shows the two applied forces that act on the crate. These two forces, plus the kinetic frictional force fk constitute the net force that acts on the crate. Once the net force has been determined, Newtons second law, F = ma (Equation 4.1) can be used to find the acceleration of the crate. SOLUTION The sum of the applied forces is F = F1 + F2. The x-component of this sum is Fx = F1 cos 55.0 + F2 = (88.0 N) cos 55.0 + 54.0 N = 104 N. The y-component of F is Fy = F1 sin 55.0 = (88.0 N) sin 55.0 = 72.1 N. The magnitude of F is F = Fx2 + Fy2 = ( 104 N ) 2 + ( 72.1 N ) 2 = 127 N 55.0 F2 Since the crate starts from rest, it moves along the direction of F. The kinetic frictional force fk opposes the motion, so it points opposite to F. The net force acting on the crate is the sum of F and fk. The magnitude a of the crates acceleration is equal to the magnitude F of the net force divided by the mass m of the crate a= F f k + F = m m (4.1) According to Equation 4.8, the magnitude fk of the kinetic frictional force is given by f k = k FN , where FN is the magnitude of the normal force. In this situation, FN is equal to 194 FORCES AND NEWTON'S LAWS OF MOTION the magnitude of the crates weight, so FN = mg. Thus, the x-component of the acceleration is ( ) 2 k mg + F ( 0.350 ) ( 25.0 kg ) 9.80 m/s + 127 N a= = = 1.65 m/s 2 m 25.0 kg The crate moves along the direction of F, whose x and y components have been determined previously. Therefore, the acceleration is also along F. The angle that F makes with the xaxis can be found using the inverse tangent function: Fy = tan 1 F x F1 sin 55.0 1 = tan F cos 55.0 + F 2 1 ( 88.0 N ) sin 55.0 = tan 1 = 34.6 above the x axis ( 88.0 N ) cos 55.0 + 54.0 N ____________________________________________________________________________________________ 52. REASONING AND SOLUTION horizontal directions gives Newtons second law applied in the vertical and L cos 21.0 W = 0 L sin 21.0 R = 0 (1) (2) a. Equation (1) gives 21.0 W 53 800 N L= = = 57 600 N cos 21.0 cos 21.0 b. Equation (2) gives L R R = L sin 21.0 = ( 57 600 N ) sin 21.0 = 20 600 N W ____________________________________________________________________________________________ 53. SSM REASONING In order for the object to move with constant velocity, the net force on the object must be zero. Therefore, the north/south component of the third force must be equal in magnitude and opposite in direction to the 80.0 N force, while the east/west component of the third force must be equal in magnitude and opposite in direction to the 60.0 N force. Therefore, the third force has components: 80.0 N due south and 60.0 N due east. We can use the Pythagorean theorem and trigonometry to find the magnitude and direction of this third force. Chapter 4 Problems SOLUTION The magnitude of the third force is 195 N F3 = (80.0 N)2 + (60.0 N) 2 = 1.00 10 2 N E The direction of F3 is specified by the angle where 80.0 N 80.0 N = tan 1 = 53.1, south of east 60.0 N F3 60.0 N ____________________________________________________________________________________________ 54. REASONING AND SOLUTION a. In the horizontal direction the thrust F is balanced by the resistive force fr of the water. That is, Fx = 0 or fr = F = 7.40 105 N b. In the vertical direction, the weight, mg, is balanced by the buoyant force, Fb. So Fy = 0 gives Fb = mg = (1.70 108 kg)(9.80 m/s2) = 1.67 109 N ____________________________________________________________________________________________ 55. REASONING The drawing shows the two forces, T and T , that act on the tooth. To obtain the net force, we will add the two 16.0 forces using the method of components (see T Section 1.8). +y +x 16.0 T SOLUTION The table lists the two vectors and their x and y components: Vector x component y component T +T cos 16.0 T sin 16.0 T T cos 16.0 T sin 16.0 T + T +T cos 16.0 T cos 16.0 T sin 16.0 T sin 16.0 196 FORCES AND NEWTON'S LAWS OF MOTION Since we are given that T = T = 21.0 N, the sum of the x components of the forces is Fx = +T cos 16.0 T cos 16.0 = +(21 N) cos 16.0 (21 N) cos 16.0 = 0 N The sum of the y components is Fy = T sin16.0 T sin 16.0 = (21 N) sin 16.0 (21 N) sin 16.0 = 11.6 N The magnitude F of the net force exerted on the tooth is F= ( Fx ) 2 + ( Fy ) 2 = ( 0 N ) 2 + ( 11.6 N ) 2 = 11.6 N _____________________________________________________________________________ _ 56. REASONING At first glance there seems to be very little information given. However, it is enough. In part a of the drawing the bucket is hanging stationary and, therefore, is in equilibrium. The forces acting on it are its weight and the two tension forces from the rope. There are two tension forces from the rope, because the rope is attached to the bucket handle at two places. These three forces must balance, which will allow us to determine the weight of the bucket. In part b of the drawing, the bucket is again in equilibrium, since it is traveling at a constant velocity and, therefore, has no acceleration. The forces acting on the bucket now are its weight and a single tension force from the rope, and they again must balance. In part b, there is only a single tension force, because the rope is attached to the bucket handle only at one place. This will allow us to determine the tension in part b, since the weight is known. SOLUTION Let W be the weight of the bucket, and let T be the tension in the rope as the bucket is being pulled up at a constant velocity. The free-body diagrams for the bucket in parts a and b of the drawing are as follows: Chapter 4 Problems 197 T 92.0 N 92.0 N W W Free-body diagram for part a Free-body diagram for part b Since the bucket in part a is in equilibrium, the net force acting on it is zero. Taking upward to be the positive direction, we have F = 92.0 N + 92.0 N W = 0 or W = 184 N Similarly, in part b we have F = T W = 0 57. or T = W = 184 N SSM REASONING AND SOLUTION The free body diagram for the plane is shown below to the left. The figure at the right shows the forces resolved into components parallel to and perpendicular to the line of motion of the plane. L L T R T R W W sin W cos If the plane is to continue at constant velocity, the resultant force must still be zero after the fuel is jettisoned. Therefore (using the directions of T and L to define the positive directions), T R W(sin ) = 0 (1) L W (cos ) = 0 (2) From Example 13, before the fuel is jettisoned, the weight of the plane is 86 500 N, the thrust is 103 000 N, and the lift is 74 900 N. The force of air resistance is the same before and after the fuel is jettisoned and is given in Example 13 as R = 59 800 N. After the fuel is jettisoned, W = 86 500 N 2800 N = 83 700 N 198 FORCES AND NEWTON'S LAWS OF MOTION From Equation (1) above, the thrust after the fuel is jettisoned is T = R + W (sin ) = [(59 800 N) + (83 700 N)(sin 30.0)] = 101 600 N From Equation (2), the lift after the fuel is jettisoned is L = W (cos ) = (83 700 N)(cos 30.0) = 72 500 N a. The pilot must, therefore, reduce the thrust by 103 000 N 101 600 N = 1400 N b. The pilot must reduce the lift by 74 900 N 72 500 N = 2400 N ____________________________________________________________________________________________ 58. REASONING The worker is standing still. Therefore, he is in equilibrium, and the net force acting on him is zero. The static frictional force fs that prevents him from slipping points upward along the roof, an angle of degrees above the horizontal; we choose this as the x direction (see the free-body diagram). The normal force FN is perpendicular to the roof and thus has no x component. But the gravitational force mg of the earth on the worker points straight down, and thus has a component parallel to the roof. We will use this freebody diagram and find the workers mass m by applying Newtons second law with the acceleration equal to zero. +y FN fs mg sin +x mg Free-body diagram of the worker SOLUTION The static frictional force fs points in the x direction, and the x component of the workers weight mg points in the +x direction. Because there are no other forces with x components, and the workers acceleration is zero, these two forces must balance each other. The x component of the workers weight is mg sin , therefore fs = mg sin . Solving this relation for the workers mass, we obtain m= fs g sin = 390 N ( 9.80 m/s2 ) ( sin 36o ) = 68 kg 59. REASONING The sum of the angles the right and left surfaces make with the horizontal and the angle between the two surface +y F1 F2 45.0 45.0 +x W Free-body diagram of the wine bottle Chapter 4 Problems 199 must be 180.0. Therefore, the angle that the left surface makes with respect to the horizontal is 180.0 90.0 45.0 = 45.0. In the free-body diagram of the wine bottle, the x axis is the horizontal. The force each surface exerts on the bottle is perpendicular to the surfaces, so both forces are directed 45.0 above the horizontal. Letting the surface on the right be surface 1, and the surface on the left be surface 2, the forces F1 and F2 are as shown in the free-body diagram. The third force acting on the bottle is W, its weight or the gravitational force exerted on it by the earth. We will apply Newtons second law and analyze the vertical forces in this free-body diagram to determine the magnitude F of the forces F1 and F2, using the fact that the bottle is in equilibrium. SOLUTION The vertical components of the forces exerted by the surfaces are F1y = F1 sin 45.0 and F2y = F2 sin 45.0. But the forces F1 and F2 have the same magnitude F, so the two vertical components become F1y = F2y = F sin 45.0. Because the bottle is in equilibrium, the upward forces must balance the downward force: F1 y + F2 y = W or F sin 45.0o + F sin 45.0o = W or 2 F sin 45.0o = W Therefore, F= W 2sin 45.0o = mg 2sin 45.0o = ( 1.40 kg ) ( 9.80 m/s 2 ) 2 sin 45.0o 60. REASONING The free-body diagram in the drawing at the right shows the forces that act on the clown (weight = W). In this drawing, note that P denotes the pulling force. Since the rope passes around three pulleys, forces of magnitude P are applied both to the clowns hands and his feet. The normal force due to the floor is FN , and the maximum static frictional force is fsMAX. At the instant just before the clowns feet move, the net vertical and net horizontal forces are zero, according to Newtons second law, since there is no acceleration at this instant. = 9.70 N FN P fsMAX P W SOLUTION According to Newtons second law, with upward and to the right chosen as the positive directions, we have F + P W = 0 1N4 4 2 4 43 Vertical forces and f MAX P = 0 1s 4 2 4 3 Horizontal forces 200 FORCES AND NEWTON'S LAWS OF MOTION From the horizontal-force equation we find P = fsMAX. But fsMAX = sFN . From the vertical-force equation, the normal force is FN = W P . With these substitutions, it follows that P = f sMAX = s FN = s ( W P ) Solving for P gives P= sW ( 0.53) ( 890 N ) = = 310 N 1 + s 1 + 0.53 ____________________________________________________________________________________________ 61. REASONING Since the boxes are at rest, they are in equilibrium. According to Equation 4.9b, the net force in the vertical, or y, direction is zero, Fy = 0. There are two unknowns in this problem, the normal force that the exerts table on box 1 and the tension in the rope that connects boxes 2 and 3. To determine these unknowns we will apply the relation Fy = 0 twice, once to the boxes on the left of the pulley and once to the box on the right. SOLUTION There are four forces acting on the two boxes on the left. The boxes are in equilibrium, so that the net force must be zero. Choosing the +y direction as being the upward direction, we have that W W2 + FN + T = 0 1 414 4 2 4 4 43 (1) Fy where W1 and W2 are the magnitudes of the weights of the boxes, FN is the magnitude of the normal force that the table exerts on box 1, and T is the magnitude of the tension in the rope. We know the weights. To find the unknown tension, note that the box 3 is also in equilibrium, so that the net force acting on it must be zero. W3 + T = 0 1 4 2 43 Fy so that T 3 T FN W3 2 1 T = W3 Substituting this expression for T into Equation (1) and solving for the normal force gives W2 W1 FN = W1 + W2 W3 = 55 N + 35 N 28 N = 62 N ____________________________________________________________________________________________ 62. REASONING Since the mountain climber is at rest, she is in equilibrium and the net force acting on her must be zero. Three forces comprise the net force, her weight, and the tension forces from the left and right sides of the rope. We will resolve the forces into components and set the sum of the x components and the sum of the y components separately equal to Chapter 4 Problems 201 zero. In so doing we will obtain two equations containing the unknown quantities, the tension TL in the left side of the rope and the tension TR in the right side. These two equations will be solved simultaneously to give values for the two unknowns. SOLUTION Using W to denote the weight of the mountain climber and choosing right and upward to be the positive directions, we have the following free-body diagram for the climber: For the x components of the forces we have Fx = TR sin 80.0 TL sin 65.0 = 0 +y TL 65.0 80.0 For the y components of the forces we have Fy = TR cos80.0 + TL cos 65.0 W = 0 TR +x W Solving the first of these equations for TR, we find that TR = TL sin 65.0 sin 80.0 Substituting this result into the second equation gives TL sin 65.0 cos80.0 + TL cos 65.0 W = 0 sin 80.0 or TL = 1.717 W Using this result in the expression for TR reveals that TR = TL sin 65.0 sin 65.0 = ( 1.717W ) = 1.580 W sin 80.0 sin 80.0 Since the weight of the climber is W = 535 N, we find that TL = 1.717 W = 1.717 ( 535 N ) = 919 N TR = 1.580 W = 1.580 ( 535 N ) = 845 N 63. SSM REASONING There are four forces that act on the chandelier; they are the forces of tension T in each of the three wires, and the downward force of gravity mg. Under the influence of these forces, the chandelier is at rest and, therefore, in equilibrium. Consequently, the sum of the x components as well as the sum of the y components of the forces must each be zero. The figure below shows a quasi-free-body diagram for the 202 FORCES AND NEWTON'S LAWS OF MOTION chandelier and the force components for a suitable system of x, y axes. Note that the diagram only shows one of the forces of tension; the second and third tension forces are not shown in the interest of clarity. The triangle at the right shows the geometry of one of the cords, where l is the length of the cord, and d is the distance from the ceiling. y T d x l mg We can use the forces in the y direction to find the magnitude T of the tension in any one wire. SOLUTION that Remembering that there are three tension forces, we see from the diagram 3T sin = mg or T= mg mg mg l = = 3 sin 3(d / l) 3d Therefore, the magnitude of the tension in any one of the cords is T= (44 kg)(9.80 m/s 2 )(2.0 m) = 1.9 102 N 3(1.5 m) ____________________________________________________________________________________________ 64. REASONING The diagram at the right shows the force F that F the ground exerts on the end of a crutch. This force, as mentioned in the statement of the problem, acts along the crutch and, therefore, makes an angle with respect to the vertical. The horizontal and vertical components of this force are also shown. The horizontal component, F sin , is the static F sin frictional force that prevents the crutch from slipping on the floor, so fs = F sin . The largest value that the static frictional F cos force can have before the crutch begins to slip is then given by fsMAX = F sin MAX . We also know from Section 4.9 (see Equation 4.7) that the maximum static frictional force is related to the magnitude FN of the normal force by fsMAX = s FN , where s is the coefficient of static friction. These two relations will allow us to find MAX . SOLUTION The magnitude of the maximum static frictional force is given by fsMAX = s FN . But, as mentioned in the REASONING section, fsMAX is also the Chapter 4 Problems 203 horizontal component of the force F, so fsMAX = F sin MAX . The vertical component of F is F cos MAX and is the magnitude FN of the normal force that the ground exerts on the crutch. Thus, we have f MAX = s FN { 1s 2 4 43 F cos MAX F sin MAX The force F can be algebraically eliminated from this equation, leaving sin MAX = s cos MAX or tan MAX = s The maximum angle that a crutch can have is MAX = tan 1 ( s ) = tan 1 ( 0.90 ) = 42 _____________________________________________________________________________ _ 65. REASONING The toboggan has a constant velocity, so it has no acceleration and is in equilibrium. Therefore, the forces acting on the toboggan must balance, that is, the net force acting on the toboggan must be zero. There are three forces present, the kinetic frictional force, the normal force from the inclined surface, and the weight mg of the toboggan. Using Newtons second law with the acceleration equal to zero, we will obtain the kinetic friction coefficient. SOLUTION In drawing the free-body diagram for the toboggan we choose the + x axis to be parallel to the hill surface and downward, the +y direction being perpendicular to the hill surface. We also use fk to symbolize the frictional force. Since the toboggan is in equilibrium, the zero net force components in the x and y directions are +y FN fk 8.00 +x mg 8.00 Fx = mg sin 8.00 k FN = 0 Fy = FN mg cos8.00 = 0 In the first of these expressions we have used Equation 4.8 for fk to express the kinetic frictional force. Solving the second equation for the normal force FN and substituting into the first equation gives 204 FORCES AND NEWTON'S LAWS OF MOTION mg sin 8.00 k mg cos8.00 = 0 1 4 2 44 4 3 F or k = sin 8.00 = tan 8.00 = 0.141 cos8.00 N 66. REASONING The block is in equilibrium in each case. Since the block moves at a constant velocity in each case, it is not accelerating. A zero acceleration is the hallmark of equilibrium. At equilibrium, the net force is zero (i.e., the forces balance to zero), and we will obtain the magnitude of the pushing force by utilizing this fact as it pertains to the vertical or y ( ) direction. We will use Equation 4.9b Fy = 0 for this purpose. +y +y It is important to note, however, that the direction of the kinetic frictional force is +x +x not the same in each case. The frictional force always opposes the fk relative motion between the surface of fk the block and the wall. Therefore, when P P the block slides upward, the frictional W W force points downward. When the block Free-body diagram for Free-body diagram for slides downward, the frictional force upward motion of the downward motion of points upward. These directions are block the block shown in the free-body diagrams (not to scale) for the two cases. In these drawings W is the weight of the block and fk is the kinetic frictional force. In each case the magnitude of the frictional force is the same. It is given by Equation 4.8 as fk = kFN, where k is the coefficient of kinetic friction and FN is the magnitude of the normal force. The coefficient of kinetic friction does not depend on the direction of the motion. Furthermore, the magnitude of the normal force in each case is the component of the pushing force that is perpendicular to the wall, or FN = P sin . SOLUTION Using Equations 4.9b to describe the balance of forces that act on the block in the y direction and referring to the free-body diagrams, we have Upward motion Fy = P cos W f k = 0 Downward motion Fy = P cos W + f k = 0 According to Equation 4.8, the magnitude of the kinetic frictional force is fk = kFN, where we have pointed out in the REASONING that the magnitude of the normal force is FN = P sin . Substituting into the equations for Fy in the two cases, we obtain Chapter 4 Problems Upward motion Fy = P cos W k P sin = 0 Downward motion 205 Fy = P cos W + k P sin = 0 Solving each case for P, we find that a. Upward motion P= W 39.0 N = = 52.6 N cos k sin cos 30.0 ( 0.250 ) sin 30.0 b. Downward motion P= 67. W 39.0 N = = 39.4 N cos + k sin cos 30.0 + ( 0.250 ) sin 30.0 SSM REASONING When the bicycle is coasting straight down the hill, the forces that act on it are the normal force FN exerted by the surface of the hill, the force of gravity mg, and the force of air resistance R. When the bicycle climbs the hill, there is one additional force; it is the applied force that is required for the bicyclist to climb the hill at constant speed. We can use our knowledge of the motion of the bicycle down the hill to find R. Once R is known, we can analyze the motion of the bicycle as it climbs the hill. SOLUTION The figure to the left below shows the free-body diagram for the forces during the downhill motion. The hill is inclined at an angle above the horizontal. The figure to the right shows these forces resolved into components parallel to and perpendicular to the line of motion. +y +x FN FN R R mg sin mg mg cos Since the bicyclist is traveling at a constant velocity, his acceleration is zero. Therefore, according to Newton's second law, we have Fx = 0 and Fy = 0 . Taking the direction up the hill as positive, we have Fx = R mg sin = 0 , or R = mg sin = (80.0 kg)(9.80 m / s 2 ) sin 15.0 = 203 N 206 FORCES AND NEWTON'S LAWS OF MOTION When the bicyclist climbs the same hill at constant speed, an applied force P must push the system up the hill. Since the speed is the same, the magnitude of the force of air resistance will remain 203 N. However, the air resistance will oppose the motion by pointing down the hill. The figure at the right shows the resolved forces that act on the system during the uphill motion. F N P mg sin mg cos R Using the same sign convention as above, we have Fx = P mg sin R = 0 , or P = R + mg sin = 203 N + 203 N = 406 N ____________________________________________________________________________________________ 68. REASONING Because the kite line is straight, the distance between the kite and the person holding the line is L = 43 m, as shown in the right part of the drawing. In order to find the height h of the kite, we need to know the angle that the kite line makes with the horizontal (see the drawing). Once that is known, h = L sin will give the kites height relative to the person. The key to finding the angle is the realization that the tension force T exerted on the kite by the line is parallel to the line itself. Therefore, the tension T is directed at an angle below the horizontal (see the free-body diagram below). To find the angle , it is sufficient, then, to find one of the components of the tension force, either Tx or Ty, because the magnitude T of the tension is known. With values for Tx or Ty and T, we can use either the sine or cosine function to determine . Both the winds force f and the tension T have horizontal and vertical components, while the weight force W is purely vertical. Therefore, there are only two horizontal forces acting on the kite (Tx, fx), but three vertical forces (Ty, fy, W), so it will be easier to calculate Tx, the horizontal component of the tension. Because the kite is stationary, the horizontal component of the tension must balance the horizontal component of the force exerted on the kite by the wind: Tx = f x (1) Chapter 4 Problems 207 f +y fy L Tx 56 fx +x W T h Ty Free-body diagram of the kite Height of the kite SOLUTION From the free-body diagram of the kite, we can see that the x components of the tension and air resistance forces are Tx = T cos and fx = f cos 56. Substituting these expressions into Equation (1), we find that Tx = f x or T cos = f cos 56o or cos = f cos 56o T o f cos 56o 1 ( 19 N ) cos 56 o = cos 1 = 48 = cos T 16 N Therefore, the kites height relative to the person is h = L sin = ( 43 m ) sin 48o = 32 m 69. REASONING The weight of the part of the washcloth off the table is moff g. At the instant just before the washcloth begins to slide, this weight is supported by a force that has magnitude equal to fsMAX, which is the static frictional force that the table surface applies to the part of the washcloth on the table. This force is transmitted around the bend in the washcloth hanging over the edge by the tension forces between the molecules of the washcloth, in much the same way that a force applied to one end of a rope is transmitted along the rope as it passes around a pulley. SOLUTION Since the static frictional supports the weight of the washcloth off the table, we have fsMAX = moff g. The static frictional force is fsMAX = sFN . The normal force FN is applied by the table to the part of the washcloth on the table and has a magnitude equal to the weight of that part of the washcloth. This is so, because the table is assumed to be 208 FORCES AND NEWTON'S LAWS OF MOTION horizontal and the part of the washcloth on it does not accelerate in the vertical direction. Thus, we have f sMAX = s FN = s mon g = moff g The magnitude g of the acceleration due to gravity can be eliminated algebraically from this result, giving smon = moff . Dividing both sides by mon + moff gives mon s mon + moff moff = mon + moff or s f on = f off where we have used fon and foff to denote the fractions of the washcloth on and off the table, respectively. Since fon + foff = 1, we can write the above equation on the left as ( ) s 1 f off = f off or f off = s 0.40 = = 0.29 1 + s 1 + 0.40 ____________________________________________________________________________________________ 70. REASONING In addition to the upward buoyant force B and the downward resistive force R, a downward gravitational force mg acts on the submarine (see the free-body diagram), where m denotes the mass of the submarine. Because the submarine is not in contact with any rigid surface, no normal force is exerted on it. We will calculate the acceleration of the submarine from Newtons second law, using the free-body diagram as a guide. +y B R SOLUTION Choosing up as the positive direction, we sum the forces mg acting on the submarine to find the net force F. According to Free-body diagram Newtons second law, the acceleration a is of the submarine ( ) 2 F B R mg 16 140 N 1030 N ( 1450 kg ) 9.80 m/s a= = = = +0.62 m/s 2 m m 1450 kg where the positive value indicates that the direction is upward. 71. REASONING According to Newtons second law, the acceleration has the same direction as the net force and a magnitude given by a = F/m. SOLUTION Since the two forces are perpendicular, the magnitude of the net force is given by the Pythagorean theorem as F = ( 40.0 N ) 2 + ( 60.0 N ) 2 . Thus, according to Newtons second law, the magnitude of the acceleration is Chapter 4 Problems F a= = m ( 40.0 N ) 2 + ( 60.0 N ) 2 4.00 kg 209 = 18.0 m/s 2 The direction of the acceleration vector is given by 60.0 N = tan 1 = 56.3 above the +x axis 40.0 N _____________________________________________________________________________ _ 72. REASONING a. Since the fish is being pulled up at a constant speed, it has no acceleration. According to Newtons second law, the net force acting on the fish must be zero. We will use this fact to determine the weight of the heaviest fish that can be pulled up. b. When the fish has an upward acceleration, we can still use Newtons second law to find the weight of the heaviest fish. However, because the fish has an acceleration, we will see that the maximum weight is less than that in part (a). SOLUTION a. There are two forces acting on the fish (taking the upward vertical direction to be the + y direction): the maximum force of +45 N due to the line, and the weight W of the fish (negative, because the weight points down). Newtons second law ( Fy = 0, Equation 4.9b ) gives Fy = +45 N W = 0 b. Since the fish has ( Fy = ma y , Equation 4.2b ) an upward or W = 45 N acceleration ay, Newtons second law becomes Fy = +45 N W = ma y Where W is the weight of the heaviest fish that can be pulled up with an acceleration. Solving this equation for W gives W = +45 N ma y The mass m of the fish is the magnitude W of its weight divided by the magnitude g of the acceleration due to gravity (see Equation 4.5), or m = W /g. Substituting this relation for m into the previous equation gives 210 FORCES AND NEWTON'S LAWS OF MOTION W W = +45 N a y g Solving this equation for W yields 45 N 45 N = = 37 N ay 2.0 m/s 2 1+ 1+ g 9.80 m/s 2 ______________________________________________________________________________ W = 73. SSM REASONING If we assume that the acceleration is constant, we can use Equation 2.4 ( v = v0 + at ) to find the acceleration of the car. Once the acceleration is known, Newton's second law ( F = ma ) can be used to find the magnitude and direction of the net force that produces the deceleration of the car. SOLUTION The average acceleration of the car is, according to Equation 2.4, a= v v0 t = 17.0 m/s 27.0 m/s = 1.25 m/s 2 8.00 s where the minus sign indicates that the direction of the acceleration is opposite to the direction of motion; therefore, the acceleration points due west. According to Newton's Second law, the net force on the car is F = ma = (1380 kg)(1.25 m/s 2 ) = 1730 N The magnitude of the net force is 1730 N . From Newton's second law, we know that the direction of the force is the same as the direction of the acceleration, so the force also points due west . ____________________________________________________________________________________________ 74. REASONING In the absence of air resistance, the two forces acting on the sensor are its weight W and the tension T in the towing cable (see the free-body diagram). We see that Tx is the only horizontal force acting on the sensor, and therefore Newtons second law Fx = max (Equation 4.2a) gives Tx = max. Because the vertical component of the sensors acceleration is zero, the vertical component of the cables tension T must balance the sensors weight: Ty = W = mg. We thus have sufficient information to calculate the horizontal and vertical +y T Ty +x T x = m ax W Free-body diagram of the sensor Chapter 4 Problems 211 components of the tension force T, and therefore to calculate its magnitude T from the 2 2 2 Pythagorean theorem: T = Tx + Ty . SOLUTION Given that Tx = max and that Ty = mg, the Pythagorean theorem yields the magnitude T of the tension in the cable: T = Tx2 + Ty2 = = (129 kg) 75. ( ma ) 2 + ( mg ) 2 = m2 a 2 + m2 g 2 = m a 2 + g 2 ( 2.84 m/s2 ) + ( 9.80 m/s2 ) 2 2 = 1320 N SSM REASONING AND SOLUTION a. Each cart has the same mass and acceleration; therefore, the net force acting on any one of the carts is, according to Newton's second law F = ma = (26 kg)(0.050 m/s 2 ) = 1.3 N b. The fifth cart must essentially push the sixth, seventh, eight, ninth and tenth cart. In other words, it must exert on the sixth cart a total force of F = ma = 5(26 kg)(0.050 m/s 2 )= 6.5 N ____________________________________________________________________________________________ 76. REASONING AND SOLUTION Newton's second law applied to Object 1 object 1 (422 N) gives Object 2 N1 T = m1a1 T T Similarly, for object 2 (185 N) W1 T m2g = m2a2 W 2 If the string is not to break or go slack, both objects must have accelerations of the same magnitude. Then a1 = a and a2 = a. The above equations become T = m1a T m2g = m2a (1) (2) 212 FORCES AND NEWTON'S LAWS OF MOTION a. Substituting Equation (1) into Equation (2) and solving for a yields a= m2 g m1 + m2 The masses of objects 1 and 2 are m1 = W1 / g = ( 422 N ) / ( 9.80 m/s 2 ) = 43.1 kg m2 = W2 / g = ( 185 N ) / ( 9.80 m/s 2 ) = 18.9 kg The acceleration is a= m2 g ( 18.9 kg ) ( 9.80 m/s 2 ) = = 2.99 m/s 2 m1 + m2 43.1 kg + 18.9 kg b. Using this value in Equation (1) gives T = m1a = ( 43.1 kg ) ( 2.99 m/s 2 ) = 129 N ____________________________________________________________________________________________ 77. F N SSM WWW REASONING The speed of the skateboarder at the bottom of the ramp can be found by solving Equation 2.9 2 ( v 2 = v0 + 2ax , where x is the distance that the skater moves mgsin mgcos down the ramp) for v. The figure at the right shows the freebody diagram for the skateboarder. The net force F, which accelerates the skateboarder down the ramp, is the component of the weight that is parallel to the incline: F = mg sin . Therefore, we know from Newton's second law that the acceleration of the skateboarder down the ramp is a= F mg sin = = g sin m m SOLUTION Thus, the speed of the skateboarder at the bottom of the ramp is 2 2 v = v0 + 2ax = v0 + 2 gx sin = (2.6 m/s) 2 + 2(9.80 m/s 2 )(6.0 m) sin18 = 6.6 m/s _____________________________________________________________________________ _ Chapter 4 Problems 213 78. REASONING AND SOLUTION From Newton's second law and the equation: v = v0 + at, we have v v0 F = ma = m t a. When the skier accelerates from rest (v0 = 0 m/s) to a speed of 11 m/s in 8.0 s, the required net force is F =m v v0 t = (73 kg) (11 m/s) 0 m/s = 1.0 102 N 8.0 s b. When the skier lets go of the tow rope and glides to a halt ( v = 0 m/s) in 21 s, the net force acting on the skier is F =m v v0 t = (73 kg) 0 m/s (11 m/s) = 38 N 21 s The magnitude of the net force is 38 N . ____________________________________________________________________________________________ 79. REASONING The only horizontal force acting on the boat and trailer is the tension in the hitch; therefore, it is the net force. According to Newtons second law, the tension (or the net force) equals the mass times the acceleration. The mass is known, and the acceleration can be found by applying an appropriate equation of kinematics from Chapter 3. SOLUTION Assume that the boat and trailer are moving in the + x direction. Newtons second law is Fx = ma x (see Equation 4.2a), where the net force is just the tension + T in the hitch, so Fx = T . Thus, T = ma x (1) Since the initial and final velocities, v0x and vx, and the time t are known, we may use Equation 3.3a from the equations of kinematics to relate these variables to the acceleration: vx = v0 x + ax t (3.3a) Solving Equation (3.3a) for ax and substituting the result into Equation (1), we find that v v T = ma x = m x 0 x t 11 m/s 0 m/s = ( 410 kg ) = 160 N 28 s ____________________________________________________________________________________________ 214 FORCES AND NEWTON'S LAWS OF MOTION 80. REASONING Since we assume that there is no +y frictional force resisting the airplanes motion, the only horizontal force acting on the airplane arises because of the tension (magnitude = T) in the cable. From Newtons FN second law F = Ma (Equation 4.1), we conclude that the airplanes acceleration is given by a = F/M = T/M, fsMAX T where M is the mass of the airplane. The harder the man +x pulls on the cable, the greater the tension T, and the greater the airplanes acceleration. According to Newtons third law, however, the cable also exerts an mg opposing horizontal force of magnitude T on the man. Thus, if he is to keep his footing, T cannot exceed the Free-body diagram maximum force of static friction fsMAX the runway of the man exerts on him. Therefore, the airplanes acceleration is greatest when these two forces have equal magnitudes: T = fsMAX (see the free-body diagram of the man). The maximum static frictional force the runway can exert is determined by the relation fsMAX = s FN (Equation 4.7). Because the man has no acceleration in the vertical direction, the normal force must balance the downward pull of gravity: FN = mg. SOLUTION Combining FN = mg and fsMAX = s FN (Equation 4.7), we obtain the maximum tension in the cable: T = fsMAX = s FN = s mg (1) We can now substitute Equation (1) for the tension into Newtons second law ( a = T/M), and calculate the maximum possible acceleration of the airplane: ( ) 2 T s mg ( 0.77 ) ( 85 kg ) 9.80 m/s a= = = = 5.9 103 m/s 2 M M 109 000 kg 81. REASONING AND SOLUTION If the +x axis is taken to be parallel to and up the ramp, then Fx = max gives T fk mg sin 30.0 = max where fk = kFN . Hence, T = max + kFN + mg sin 30.0 Also, Fy = may gives FN mg cos 3 0.0 = 0 (1) Chapter 4 Problems 215 since no acceleration occurs in this direction. Then FN = mg cos 3 0.0 (2) Substitution of Equation (2) into Equation (1) yields T = max + kmg cos 30.0 + mg sin 30.0 T = (205 kg)(0.800 m/s2) + (0.900)(205 kg)(9.80 m/s2)cos 30.0 + (205 kg)(9.80 m/s2)sin 30.0 = 2730 N ____________________________________________________________________________________________ 82. REASONING To determine the mans upward acceleration by + means of Newtons second law, we first need to identify all of the forces exerted on him and then construct a free-body diagram. The earth pulls down on the man with a gravitational force W = mg. Once T T he begins accelerating upward, he is no longer in contact with the ground, so there is no normal force acting on him. The pulling force P that he exerts on the rope does not appear in his free-body diagram because it is not a force exerted on him. Each end of the rope exerts a W tension force on him. If we assume that the rope is massless, and ignore friction between the rope and the branch, then the magnitude of the tension T is the same everywhere in the rope. Because the man Free-body diagram of the man pulls down on the free end intentionally and on the other end inadvertently (because it is tied around his waist), Newtons third law predicts that both ends of the rope pull upward on him. The third law predicts that the free end of the rope pulls up on the man with a force exactly equal in magnitude to that of the 358-N pulling force. Thus, in addition to the downward gravitational force, there are two upward tension forces with magnitudes T = 358 N acting on the man, as illustrated in the free-body diagram. SOLUTION Taking up as the positive direction and applying Newtons second law to the mans free-body diagram yields F = 2T W = ma Solving for the acceleration a, we find a= 2 ( 358 N ) 2T W 2T mg 2T = = g = 9.80 m/s 2 = 0.14 m/s 2 m m m 72.0 kg 216 FORCES AND NEWTON'S LAWS OF MOTION 83. SSM REASONING The free-body diagrams for Robin (mass = m) and for the chandelier (mass = M) are given at the right. The tension T in the rope applies an Robin upward force to both. Robin accelerates upward, while the chandelier accelerates downward, each acceleration having the same magnitude. Our solution is based on separate applications of Newtons second law to Robin and the chandelier. T T Chandelier mg Mg SOLUTION Applying Newtons second law, we find T mg = ma 1 4 2 44 4 3 T Mg = Ma 1 44 2 4 43 and Robin Hood Chandelier In these applications we have taken upward as the positive direction, so that Robins acceleration is a, while the chandeliers acceleration is a. Solving the Robin-Hood equation for T gives T = mg + ma Substituting this expression for T into the Chandelier equation gives mg + ma Mg = Ma M m a = g M +m or a. Robins acceleration is ( 195 kg ) ( 77.0 kg ) M m 2 2 a= 9.80 m/s = 4.25 m/s g = M +m ( 195 kg ) + ( 77.0 kg ) ( ) b. Substituting the value of a into the expression for T gives ( ) T = mg + ma = ( 77.0 kg ) 9.80 m/s 2 + 4.25 m/s 2 = 1080 N ____________________________________________________________________________________________ 84. REASONING Newtons second law, Equation 4.2a, can be used to find the tension in the coupling between the cars, since the mass and acceleration are known. The tension in the coupling between the 30th and 31st cars is responsible for providing the acceleration for the 20 cars from the 31st to the 50th car. The tension in the coupling between the 49th and 50th cars is responsible only for pulling one car, the 50th. SOLUTION a. The tension T between the 30th and 31st cars is Chapter 4 Problems Tx = (Mass of 20 cars) ax 217 (4.2a) ( )( ) = ( 20 cars ) 6.8 103 kg/car 8.0 102 m / s 2 = 1.1 10 4 N b. The tension T between the 49th and 50th cars is Tx = (Mass of 1 car)ax ( (4.2a) )( ) = ( 1 car ) 6.8 103 kg/car 8.0 10 2 m / s 2 = 5.4 102 N ____________________________________________________________________________________________ 85. REASONING The box comes to a halt because the kinetic F N frictional force and the component of its weight parallel to the incline oppose the motion and cause the box to slow down. The distance that the box travels up the incline can be can be found mg sin 2 by solving Equation 2.9 ( v 2 = v0 + 2ax ) for x. Before we use this approach, however, we must first determine the acceleration fk of the box as it travels along the incline. mg cos SOLUTION The figure above shows the free-body diagram for the box. It shows the resolved components of the forces that act on the box. If we take the direction up the incline as the positive x direction, then, Newton's second law gives Fx = mg sin f k = max mg sin k FN = ma x or where we have used Equation 4.8, f k = k FN . In the y direction we have Fy = FN mg cos = 0 or FN = mg cos since there is no acceleration in the y direction. Therefore, the equation for the motion in the x direction becomes mg sin k mg cos = ma x or a x = g (sin + k cos ) According to Equation 2.9, with this value for the acceleration and the fact that v = 0 m/s, the distance that the box slides up the incline is x= 2 v0 2a = 2 v0 2 g (sin + k cos ) = (1.50 m/s) 2 = 0.265 m 2(9.80 m/s 2 )[sin 15.0 + (0.180)cos 15.0] ____________________________________________________________________________________________ 218 FORCES AND NEWTON'S LAWS OF MOTION 86. REASONING Since we assume that there is no frictional force resisting the airplanes motion, the only horizontal force acting on the airplane arises because of the tension (magnitude = T) in the cable. The airplane (mass = M) undergoes a horizontal acceleration caused by the horizontal component Tx = T cos of the tension force, where is the angle that the cable makes with the horizontal. From Newtons second law, the acceleration of the airplane is ax = Fx M = T cos M +y FN fsMAX T Tx +x Ty mg (1) Free-body diagram of the man The maximum tension in the cable is limited by the condition that the mans feet must not slip. When the man pulls as hard as possible without slipping, the horizontal component of the tension acting on him matches the maximum static frictional force: Tx = T cos = fsMAX (see the free-body diagram of the man). The maximum static frictional force itself is given by fsMAX = s FN (Equation 4.7). Together, these two relations yield T cos = s FN (2) To evaluate the magnitude FN of the normal force that acts on the man, we must consider Newtons third law. This law indicates that when the man (mass = m) pulls up on the cable, the cable pulls down on him (see the free-body diagram of the man). This additional downward force increases the upward normal force FN the runway exerts on him. Applying Newtons second law to the vertical direction in this diagram, with zero acceleration, we see that Fy = FN Ty mg = FN T sin mg = 0 . Solving for FN yields FN = T sin + mg (3) Substituting Equation (3) into Equation (2) yields an expression in which the tension T is the only unknown quantity: T cos = s ( T sin + mg ) (4) We now solve Equation (4) for the tension in the cable: T cos = sT sin + s mg or T cos sT sin = s mg T= s mg cos s sin or T ( cos s sin ) = s mg (5) Chapter 4 Problems 219 Equation (5) may be substituted into Equation (1) for the airplanes acceleration: ax = s mg cos T cos = M M ( cos s sin ) (6) SOLUTION We apply Equation (6) to calculate the acceleration of the airplane: ( ) ( 0.77 ) ( 85 kg ) 9.80 m/s 2 cos 9.0o s mg cos ax = = = 6.7 103 m/s 2 o o M ( cos s sin ) ( 109 000 kg ) cos 9.0 0.77 sin 9.0 ( ) 87. REASONING As the free-body diagram shows, there are two forces acting on the fireman as he slides down the pole: his weight W and the kinetic frictional force fk. The kinetic frictional force opposes the motion of the fireman, so it points upward in the +y direction. In accord with Newtons second law, the net force, which is the sum of these two forces, is equal to the firemans mass times his acceleration. His mass, and therefore his weight, is known, but his acceleration is not. We will turn to one of the equations of kinematics from Chapter 3 to determine the acceleration. SOLUTION Newtons second law ( Fy = ma y , Equation 4.2b ) can +y fk W Free-body diagram for the fireman be applied to this situation: Fy = + f k W = ma y The magnitude W of the firemans weight can be expressed in terms of his mass as W = mg (Equation 4.5), where g is magnitude of the acceleration due to gravity. Solving the equation above for the magnitude of the kinetic frictional force, and using W = mg, gives f k = ma y + W = ma y + mg (1) Since the initial and final velocities, v0y and vy, and the displacement y are known, we will use Equation 3.6b from the equations of kinematics to relate these variables to the 2 2 acceleration: v y = v0 y + 2a y y . Solving this equation for ay and substituting the result into Equation 1 gives 2 v 2 v0 y y fk = m 2y + mg 220 FORCES AND NEWTON'S LAWS OF MOTION We note that the fireman slides down the pole, so his displacement is negative, or y = 4.0 m. The magnitude of the kinetic frictional force is, then, 2 v 2 v0 y y fk = m 2y + mg ( 1.4 m/s ) 2 ( 0 m/s ) 2 = ( 86 kg ) + ( 86 kg ) ( 9.80 m/s 2 ) = 820 N ( 4.0 m ) 2 _____________________________________________________________________________ _ 88. REASONING Consider the forces that act on each block. Only one + force contributes to the horizontal net force acting on block 1, as shown in the free-body diagram. This is the P force P with which block 2 pushes on block 1. The minus sign in the free-body diagram indicates the Free-body diagram for direction of the force is to the left. This force is part of block 1 the action-reaction pair of forces that is consistent with Newtons third law. Block 1 pushes forward and to the right against block 2, and block 2 pushes backward and to the left against block 1 with an oppositely directed force of equal magnitude. Two forces contribute to the horizontal net force acting + on block 2, as shown in the free-body diagram. One is the force P with which block 1 pushes on block 2. P fk According to Newtons third law, this force has the same magnitude but the opposite direction as the force Free-body diagram for with which block 2 pushes on block 1. The other force block 2 is the kinetic frictional force fk, which points to the left, in opposition to the relative motion between the block and the surface on which it slides. Both blocks decelerate, the magnitude of the deceleration being the same for each block. They have the same deceleration, because they are pressed together. Since the blocks are moving to the right in the drawing, the acceleration vector points to the left, for it reflects the slowing down of the motion. In Case A and in Case B we will apply Newtons second law separately to each block in order to relate the net force to the acceleration. SOLUTION Referring to the free-body diagram for block 1, we write Newtons second law as follows: P = m1 ( a ) { (1) Net force on block 1 where a is the magnitude of the acceleration. The minus sign appears on the right side of this equation because the acceleration, being a deceleration, points to the left, in the negative Chapter 4 Problems 221 direction. Referring to the free-body diagram for block 2, we write Newtons second law as follows: P f = m2 ( a ) 1 2 3k (2) Net force on block 2 Solving Equation (1) for a gives a = P / m1 . Substituting this result into Equation (2) gives P P f k = m2 m 1 or P= m1 f k m1 + m2 Substituting this result for P into a = P / m1 gives a= m1 f k fk P = = m1 m1 ( m1 + m2 ) m1 + m2 We can now use these results to calculate P and a in both cases. a. Case A P= m1 f k ( 3.0 kg ) ( 5.8 N ) = = 2.9 N m1 + m2 3.0 kg + 3.0 kg Case B P= m1 f k ( 6.0 kg ) ( 5.8 N ) = 3.9 N = m1 + m2 6.0 kg + 3.0 kg b. Case A a= fk 5.8 N = = 0.97 m/s 2 m1 + m2 3.0 kg + 3.0 kg The magnitude of the acceleration is 0.97 m/s 2 . Case B a= fk 5.8 N = = 0.64 m/s 2 m1 + m2 6.0 kg + 3.0 kg The magnitude of the acceleration is 0.64 m/s 2 . 89. SSM REASONING The tension in each coupling bar is responsible for accelerating the objects behind it. The masses of the cars are m1, m2, and m3. We can use Newtons second law to express the tension in each coupling bar, since friction is negligible: 222 FORCES AND NEWTON'S LAWS OF MOTION TA = ( m1 + m2 + m3 ) a 1 4 4 42 4 4 43 TB = ( m2 + m3 ) a 1 4 42 4 43 Coupling bar A TC = m3a 14 2 4 3 Coupling bar C Coupling bar B In these expressions a = 0.12 m/s2 remains constant. Consequently, the tension in a given bar will change only if the total mass of the objects accelerated by that bar changes as a result of the luggage transfer. Using (Greek capital delta) to denote a change in the usual fashion, we can express the changes in the above tensions as follows: TA = ( m1 + m2 + m3 ) a 1 4 4 4 4 2 4 4 4 4 4 4 3 Coupling bar A TB = ( m2 + m3 ) a 1 4 4 4 2 4 4 4 4 4 3 Coupling bar B TC = ( m3 ) a 1 44 2 4 43 Coupling bar C SOLUTION a. Moving luggage from car 2 to car 1 does not change the total mass m1 + m2 + m3, so (m1 + m2 + m3) = 0 kg and TA = 0 N . The transfer from car 2 to car 1 causes the total mass m2 + m3 to decrease by 39 kg, so (m2 + m3) = 39 kg and ( ) TB = ( m2 + m3 ) a = ( 39 kg ) 0.12 m/s 2 = 4.7 N The transfer from car 2 to car 1 does not change the mass m3, so m3 = 0 kg and TC = 0 N . b. Moving luggage from car 2 to car 3 does not change the total mass m1 + m2 + m3, so (m1 + m2 + m3) = 0 kg and TA = 0 N . The transfer from car 2 to car 3 does not change the total mass m2 + m3, so (m2 + m3) = 0 kg and TB = 0 N . The transfer from car 2 to car 3 causes the mass m3 to increase by 39 kg, so m3 = +39 kg and TC = ( m3 ) a = ( +39 kg ) 0.12 m/s 2 = +4.7 N ( ) ____________________________________________________________________________________________ 90. REASONING AND SOLUTION The distance required for the truck to stop is found from x= 2 v 2 v0 2a 2 ( 0 m/s ) 2 v0 = 2a Chapter 4 Problems 223 The acceleration of the truck is needed. The frictional force decelerates the crate. The maximum force that friction can supply is fsMAX = sFN = smg Newton's second law requires that fsMAX = ma so a = sg Now the stopping distance is x= 2 v0 2 s g = ( 25 m/s ) 2 2 ( 0.650 ) ( 9.80 m/s 2 ) = 49.1 m ____________________________________________________________________________________________ 91. REASONING AND SOLUTION a. Newton's second law for block 1 (10.0 kg) is T = m1a (1) Block 2 (3.00 kg) has two ropes attached each carrying a tension T. Also, block 2 only travels half the distance that block 1 travels in the same amount of time so its acceleration is only half of block 1's acceleration. Newton's second law for block 2 is then 2T m2 g = 1 m2a 2 (2) Solving Equation (1) for a, substituting into Equation (2), and rearranging gives T= 1+ 1 2 1 4 m2 g ( m2 / m1 ) = 13.7 N b. Using this result in Equation (1) yields a= T 13.7 N = = 1.37 m/s 2 m1 10.0 kg ____________________________________________________________________________________________ 92. REASONING AND SOLUTION a. The force acting on the sphere which accelerates it is the horizontal component of the tension in the string. Newton's second law for the horizontal motion of the sphere gives 224 FORCES AND NEWTON'S LAWS OF MOTION T sin = ma The vertical component of the tension in the string supports the weight of the sphere so T cos = mg Eliminating T from the above equations results in a = g tan . b. ( ) a = g tan = 9.80 m/s 2 tan 10.0 = 1.73 m/s 2 c. Rearranging the result of part a and setting a = 0 m/s2 gives = tan 1 ( a / g ) = 0 ____________________________________________________________________________________________ 93. SSM REASONING AND SOLUTION a. The left mass (mass 1) has a tension T1 pulling it up. Newton's second law gives T1 m1g = m1a (1) The right mass (mass 3) has a different tension, T3, trying to pull it up. Newton's second for it is T3 m3g = m3a (2) The middle mass (mass 2) has both tensions acting on it along with friction. Newton's second law for its horizontal motion is T3 T1 km2g = m2a (3) Solving Equation (1) and Equation (2) for T1 and T3, respectively, and substituting into Equation (3) gives ( m m1 k m2 ) g a= 3 m1 + m2 + m3 Hence, ( ) 25.0 kg 10.0 kg ( 0.100 ) ( 80.0 kg ) 9.80 m/s 2 a= = 0.60 m/s 2 10.0 kg + 80.0 kg + 25.0 kg b. From part a: Chapter 4 Problems 225 ( ) T3 = m3(g a) = ( 25.0 kg ) ( 9.80 m/s 2 0.60 m/s 2 ) = 230 N T1 = m1(g + a) = ( 10.0 kg ) 9.80 m/s 2 + 0.60 m/s 2 = 104 N ____________________________________________________________________________________________ 94. REASONING AND SOLUTION a. The static frictional force is responsible for accelerating the top block so that it does not slip against the bottom one. The maximum force that can be supplied by friction is fsMAX = sFN = sm1g Newton's second law requires that fsMAX = m1a, so a = sg The force necessary to cause BOTH blocks to have this acceleration is F = (m1 + m2)a = (m1 + m2) sg 2 F = (5.00 kg + 12.0 kg)(0.600)(9.80 m/s2) = 1.00 10 N b. The maximum acceleration that the two block combination can have before slipping occurs is F a= 17.0 kg Newton's second law applied to the 5.00 kg block is F sm1g = m1a = (5.00 kg) F 17.0 kg Hence F = 41.6 N ____________________________________________________________________________________________ 95. SSM REASONING The magnitude of the gravitational force that each part exerts on the other is given by Newtons law of gravitation as F = Gm1m2 / r 2 . To use this expression, we need the masses m1 and m2 of the parts, whereas the problem statement gives the weights W1 and W2. However, the weight is related to the mass by W = mg, so that for each part we know that m = W/g. SOLUTION The gravitational force that each part exerts on the other is 226 FORCES AND NEWTON'S LAWS OF MOTION F= Gm1m2 r2 = G ( W1 / g ) ( W2 / g ) r2 m ( 6.67 1011 N 2 / kg2 ) ( 11 000 N ) ( 3400 N ) = = 2 ( 9.80 m/s2 ) ( 12 m ) 2 1.8 10 7 N ____________________________________________________________________________________________ 96. REASONING The magnitudes of the initial (v0 = 0 m/s) and final (v = 805 m/s) velocities are known. In addition, data is given for the mass and the thrust, so that Newtons second law can be used to determine the acceleration of the probe. Therefore, kinematics Equation 2.4 (v = v0 + at) can be used to determine the time t. SOLUTION Solving Equation 2.4 for the time gives t= v v0 a Newtons second law gives the acceleration as a = (F)/m. Using this expression in Equation 2.4 gives t= ( v v0 ) ( F ) / m = m ( v v0 ) F = ( 474 kg ) ( 805 m/s 0 m/s ) 56 103 N = 6.8 106 s Since one day contains 8.64 104 s, the time is ( t = 6.8 106 s 97. 1 day ) 8.64 104 s = 79 days SSM REASONING AND SOLUTION According to Equation 3.3b, the acceleration of the astronaut is a y = (v y v0 y ) / t = v y / t . The apparent weight and the true weight of the astronaut are related according to Equation 4.6. Direct substitution gives vy FN = { + ma y = m ( g + a y ) = m g + mg { t True Apparent weight weight 45 m/s 2 = (57 kg) 9.80 m/s 2 + = 7.3 10 N 15 s ____________________________________________________________________________________________ Chapter 4 Problems 227 98. REASONING According to Newton's second law ( F = ma ), the acceleration of the object is given by a = F / m , where F is the net force that acts on the object. We must first find the net force that acts on the object, and then determine the acceleration using Newton's second law. SOLUTION The following table gives the x and y components of the two forces that act on the object. The third row of that table gives the components of the net force. Force x-Component y-Component F1 40.0 N 0N F2 (60.0 N) cos 45.0 = 42.4 N (60.0 N) sin 45.0 = 42.4 N F = F1 + F2 82.4 N 42.4 N The magnitude of F is given by the Pythagorean theorem as F = (82.4 N) 2 + (42.4) 2 = 92.7 N The angle that F makes with the +x axis is 1 42.4 N = tan = 27.2 82.4 N F 42.4 N 82.4 N According to Newton's second law, the magnitude of the acceleration of the object is a= F 92.7 N = = 30.9 m/s 2 m 3.00 kg Since Newton's second law is a vector equation, we know that the direction of the right hand side must be equal to the direction of the left hand side. In other words, the direction of the acceleration a is the same as the direction of the net force F . Therefore, the direction of the acceleration of the object is 27.2 above the +x axis . ____________________________________________________________________________________________ 99. SSM REASONING In order to start the crate moving, an external agent must supply a force that is at least as large as the maximum value fsMAX = s FN , where s is the coefficient of static friction (see Equation 4.7). Once the crate is moving, the magnitude of 228 FORCES AND NEWTON'S LAWS OF MOTION the frictional force is very nearly constant at the value f k = k FN , where k is the coefficient of kinetic friction (see Equation 4.8). In both cases described in the problem statement, there are only two vertical forces that act on the crate; they are the upward normal force FN, and the downward pull of gravity (the weight) mg. Furthermore, the crate has no vertical acceleration in either case. Therefore, if we take upward as the positive direction, Newton's second law in the vertical direction gives FN mg = 0 , and we see that, in both cases, the magnitude of the normal force is FN = mg . SOLUTION a. Therefore, the applied force needed to start the crate moving is fsMAX = s mg = (0.760)(60.0 kg)(9.80 m/s 2 ) = 447 N b. When the crate moves in a straight line at constant speed, its velocity does not change, and it has zero acceleration. Thus, Newton's second law in the horizontal direction becomes P fk = 0, where P is the required pushing force. Thus, the applied force required to keep the crate sliding across the dock at a constant speed is P = f k = k mg = (0.410)(60.0 kg)(9.80 m/s 2 ) = 241 N ____________________________________________________________________________________________ 100. REASONING Newtons second law of motion gives the relationship between the net force F and the acceleration a that it causes for an object of mass m. The net force is the vector sum of all the external forces that act on the object. Here the external forces are the drive force, the force due to the wind, and the resistive force of the water. SOLUTION We choose the direction of the drive force (due west) as the positive direction. Solving Newtons second law ( F = ma ) for the acceleration gives a= F +4100 N 800 N 1200 N = = +0.31 m/s 2 m 6800 kg The positive sign for the acceleration indicates that its direction is due west . 101. REASONING AND SOLUTION The acceleration needed so that the craft touches down with zero velocity is 2 v 2 v0 ( 18.0 m/s ) 2 a= = = 0.982 m/s 2 2s 2 ( 165 m ) Newton's second law applied in the vertical direction gives Chapter 4 Problems 229 F mg = ma Then F = m(a + g) = (1.14 104 kg)(0.982 m/s2 + 1.60 m/s2) = 29 400 N ____________________________________________________________________________________________ 102. REASONING AND SOLUTION The apparent weight is FN = mw(g + a) We need to find the acceleration a. Let T represent the force applied by the hoisting cable. Newton's second law applied to the elevator gives T (mw + me)g = (mw + me)a Solving for a gives a= T 9410 N g = 9.80 m/s 2 = 0.954 m/s 2 mw + me 60.0 kg + 815 kg Now the apparent weight is FN = (60.0 kg)(9.80 m/s2 + 0.954 m/s2) = 645 N ____________________________________________________________________________________________ 103. SSM REASONING We can use the appropriate equation of kinematics to find the acceleration of the bullet. Then Newton's second law can be used to find the average net force on the bullet. SOLUTION According to Equation 2.4, the acceleration of the bullet is a= v v0 t = 715 m/s 0 m/s = 2.86 105 m/s 2 3 2.50 10 s Therefore, the net average force on the bullet is F = ma = (15 103 kg)(2.86 105 m/s 2 ) = 4290 N ____________________________________________________________________________________________ 104. REASONING The magnitude F of the net force acting on the kayak is given by Newtons second law as F = ma (Equation 4.1), where m is the combined mass of the person and kayak, and a is their acceleration. Since the initial and final velocities, v0 and v, and the displacement x are known, we can employ one of the equations of kinematics from Chapter 2 to find the acceleration. 230 FORCES AND NEWTON'S LAWS OF MOTION ( ) SOLUTION Solving Equation 2.9 v 2 = v0 2 + 2ax from the equations of kinematics for the acceleration, we have a= v 2 v0 2 2x Substituting this result into Newtons second law gives ( 0.60 m/s ) 2 ( 0 m/s ) 2 v 2 v0 2 F = ma = m = 32 N = ( 73 kg ) 2 ( 0.41 m ) 2x _____________________________________________________________________________ _ 105. REASONING AND SOLUTION a. According to Equation 4.4, the weight of an object of mass m on the surface of Mars would be given by GM M m W= 2 RM where MM is the mass of Mars and RM is the radius of Mars. On the surface of Mars, the weight of the object can be given as W = mg (see Equation 4.5), so mg = GM M m 2 RM or g= GM M 2 RM Substituting values, we have g= (6.67 1011N m 2 /kg 2 )(6.46 10 23 kg) = 3.75 m/s 2 (3.39 106 m) 2 b. According to Equation 4.5, W = mg = (65 kg)(3.75 m/s2) = 2.4 102 N ____________________________________________________________________________________________ 106. REASONING Each particle experiences two gravitational forces, one due to each of the remaining particles. To get the net gravitational force, we must add the two contributions, taking into account the directions. The magnitude of the gravitational force that any one particle exerts on another is given by Newtons law of gravitation as F = Gm1m2 / r 2 . Thus, for particle A, we need to apply this law to its interaction with particle B and with particle C. For particle B, we need to apply the law to its interaction with particle A and with particle C. Lastly, for particle C, we must apply the law to its interaction with particle A Chapter 4 Problems 231 and with particle B. In considering the directions, we remember that the gravitational force between two particles is always a force of attraction. SOLUTION We begin by calculating the magnitude of the gravitational force for each pair of particles: FAB = FBC = FAC = GmA mB m ( 6.67 1011 N 2 / kg2 ) ( 363 kg ) ( 517 kg ) = 5.007 105 N = GmB mC m ( 6.67 1011 N 2 / kg2 ) ( 517 kg ) ( 154 kg ) = 8.497 105 N = r2 r2 Gm AmC r2 ( 0.500 m ) 2 ( 0.500 m ) 2 m ( 6.67 1011 N 2 / kg2 ) ( 363 kg ) ( 154 kg ) = 6.629 106 N = ( 0.500 m ) 2 In using these magnitudes we take the direction to the right as positive. a. Both particles B and C attract particle A to the right, the net force being FA = FAB + FAC = 5.007 10 5 N + 6.629 10 6 N = 5.67 10 5 N, right b. Particle C attracts particle B to the right, while particle A attracts particle B to the left, the net force being FB = FBC FAB = 8.497 10 5 N 5.007 10 5 N = 3.49 10 5 N, right c. Both particles A and B attract particle C to the left, the net force being FC = FAC + FBC = 6.629 10 6 N + 8.497 10 5 N = 9.16 10 5 N, left ____________________________________________________________________________________________ y 107. SSM REASONING AND SOLUTION The system is shown in the drawing. We will let m1 = 21.0 kg , and m2 = 45.0 kg . Then, m1 will move upward, and m2 will move downward. There are two forces that act on each object; they are the tension T in the cord and the weight mg of the object. The forces are shown in the free-body diagrams at the far right. y T m1 T x m2 m1g m2g 232 FORCES AND NEWTON'S LAWS OF MOTION We will take up as the positive direction. acceleration of m2 must be a. If the acceleration of m1 is a, then the From Newton's second law, we have for m1 Fy = T m1g = m1a (1) Fy = T m2 g = m2 a (2) and for m2 a. Eliminating T between these two equations, we obtain a= m2 m1 45.0 kg 21.0 kg 2 2 g = (9.80 m/s ) = 3.56 m/s m2 + m1 45.0 kg + 21.0 kg b. Eliminating a between Equations (1) and (2), we find T= 2(21.0 kg)(45.0 kg) 2 g= (9.80 m/s ) = 281 N m1 + m2 21.0 kg + 45.0 kg 2m1m2 ____________________________________________________________________________________________ 108. REASONING Static friction determines the magnitude of the applied force at which either the upper or lower block begins to slide. For the upper block the static frictional force is applied only by the lower block. For the lower block, however, separate static frictional forces are applied by the upper block and by the horizontal surface. The maximum magnitude of any of the individual frictional forces is given by Equation 4.7 as the coefficient of static friction times the magnitude of the normal force. SOLUTION We begin by drawing the free-body diagram for the lower block. max fs, from A FApplied A max B fs, from surface Free-body diagram for lower block B This diagram shows that three horizontal forces act on the lower block, the applied force, and the two maximum static frictional forces, one from the upper block and one from the horizontal surface. At the instant that the lower block just begins to slide, the blocks are in equilibrium and the applied force is balanced by the two frictional forces, with the result that Chapter 4 Problems 233 MAX MAX FApplied = fs, from A + fs, from surface (1) According to Equation 4.7, the magnitude of the maximum frictional force from the surface is MAX fs, from surface = s FN = s 2mg (2) Here, we have recognized that the normal force FN from the horizontal surface must balance the weight 2mg of both blocks. It remains now to determine the 47.0 N magnitude of the maximum frictional max fs, from B MAX force fs, from A from the upper block. A To this end, we draw the free-body B diagram for the upper block at the Free-body diagram for instant that it just begins to slip due to upper block A the 47.0-N applied force. At this instant the block is in equilibrium, so that the frictional force from the lower block B balances the 47.0-N force. Thus, MAX fs, from B = 47.0 N , and according to Equation 4.7, we have MAX fs, from B = s FN = s mg = 47.0 N Here, we have recognized that the normal force FN from the lower block must balance the weight mg of only the upper block. This result tells us that smg = 47.0 N. To determine MAX fs, from A we invoke Newtons third law to conclude that the magnitudes of the frictional forces at the A-B interface are equal, since they are action-reaction forces. MAX fs, from A = s mg . Substituting this result and Equation (2) into Equation (1) gives Thus, MAX MAX FApplied = fs, from A + fs, from surface = s mg + s 2mg = 3 ( 47.0 N ) = 141 N _____________________________________________________________________________ _ 109. REASONING AND SOLUTION If the +x axis is taken in the direction of motion, Fx = 0 gives F N F fk mg sin = 0 fk where fk = kFN Then F W 2 5 .0 234 FORCES AND NEWTON'S LAWS OF MOTION F kFN mg sin =0 (1) Also, Fy = 0 gives FN mg cos = 0 so FN = mg cos (2) Substituting Equation (2) into Equation (1) and solving for F yields F = mg( sin + k cos ) F = (55.0 kg)(9.80 m/s2)[sin 25.0 + (0.120)cos 25.0] = 286 N ____________________________________________________________________________________________ 110. REASONING Since the wire beneath the limb is at rest, it is in equilibrium and the net force acting on it must be zero. Three forces comprise the net force, the 151-N force from the limb, the 447-N tension force from the left section of the wire, and the tension force T from the right section of the wire. We will resolve the forces into components and set the sum of the x components and the sum of the y components separately equal to zero. In so doing we will obtain two equations containing the unknown quantities, which are the horizontal and vertical components of the tension force T. These two equations will be solved simultaneously to give values for the two unknowns. Knowing the components of the tension force, we can determine its magnitude and direction. SOLUTION Let Tx and Ty be the horizontal and vertical components of the tension force. The free-body diagram for the wire beneath the limb is as follows: Taking upward and to the right as the positive directions, we find for the x components of the forces that Fx = Tx ( 447 N ) cos 14.0 = 0 Tx = ( 447 N ) cos 14.0 = 434 N +y T 447 N 14.0 Tx 151 N For the y components of the forces we have Fy = Ty + ( 447 N ) sin14.0 151 N = 0 Ty = ( 447 N ) sin14.0 + 151 N = 43 N The magnitude of the tension force is Ty +x Chapter 4 Problems T = Tx2 + Ty2 = ( 434 N ) 2 + ( 43 N ) 2 235 = 436 N Since the components of the tension force and the angle are related by tan = Ty / Tx , we find that Ty 43 N = tan 1 = tan 1 = 5.7 T 434 N x 111. SSM REASONING The shortest time to pull the person from the cave corresponds to the maximum acceleration, a y , that the rope can withstand. We first determine this 2 acceleration and then use kinematic Equation 3.5b ( y = v0 y t + 1 a yt ) to find the time t. 2 SOLUTION As the person is being pulled from the cave, there are two forces that act on him; they are the tension T in the rope that points vertically upward, and the weight of the person mg that points vertically downward. Thus, if we take upward as the positive direction, Newton's second law gives Fy = T mg = ma y . Solving for a y , we have ay = T T 569 N g= g= 9.80 m/s 2 = 0.92 m/s 2 2 2 m W /g (5.20 10 N)/(9.80 m/s ) 2 Therefore, from Equation 3.5b with v0 y = 0 m/s, we have y = 1 a y t . Solving for t, we find 2 t= 2y 2(35.1 m) = = 8.7 s ay 0.92 m/s 2 ____________________________________________________________________________________________ 112. REASONING The free-body diagrams for the large cube (mass = M) and the small cube (mass = m) are shown in the following drawings. In the case of the large cube, we have omitted the weight and the normal force from the surface, since the play no role in the solution (although they do balance). fsMAX FN P FN mg In these diagrams, note that the two blocks exert a normal force on each other; the large block exerts the force FN on the smaller block, while the smaller block exerts the force FN 236 FORCES AND NEWTON'S LAWS OF MOTION on the larger block. In accord with Newtons third law these forces have opposite directions and equal magnitudes FN . Under the influence of the forces shown, the two blocks have the same acceleration a. We begin our solution by applying Newtons second law to each one. SOLUTION According to Newtons second law, we have F = P FN = Ma 1 4 44 2 4 4 4 3 FN = ma 14 2 4 3 Large block Small block Substituting FN = ma into the large-block expression and solving for P gives P = (M + m) a For the smaller block to remain in place against the larger block, the static frictional force must balance the weight of the smaller block, so that fsMAX = mg. But fsMAX is given by fsMAX = sFN , where, from the Newtons second law, we know that FN = ma. Thus, we have sma = mg or a = g/s . Using this result in the expression for P gives P = ( M + m) ( M + m ) g = ( 25 kg + 4.0 kg ) ( 9.80 m/s a= s 2 0.71 )= 4.0 102 N ____________________________________________________________________________________________ 113. REASONING According to Newtons second law, the acceleration of the probe is a = F/m. Using this value for the acceleration in Equation 2.8 and noting that the probe starts from rest (v0 = 0 m/s), we can write the distance traveled by the probe as 1 F 2 x = v0t + 1 at 2 = t 2 2 m This equation is the basis for our solution. SOLUTION Since each engine produces the same amount of force or thrust T, the net force is F = 2T when the engines apply their forces in the same direction and F = T 2 + T 2 = 2T when they apply their forces perpendicularly. Thus, we write the distances traveled in the two situations as follows: 1 2T x= 2 1 44 m 24 2 t 3 4 Engines fired in same direction and 1 2T 2 x= t 2 m 1 4 4 2 4 43 Engines fired perpendicularly Chapter 4 Problems 237 Since the distances are the same, we have 2 1 2T 2 t t = 2 m 2 m 1 2T 2 2 t 2 = t or The firing time when the engines apply their forces perpendicularly is, then, t = ( 4 2 ) t = ( 4 2 ) ( 28 s ) = 33 s ____________________________________________________________________________________________ 114. REASONING The drawing shows the point between the earth and the moon where the gravitational force exerted on the spacecraft by the earth balances that exerted by the moon. The magnitude of the gravitational force exerted on the spacecraft by the earth is Fearth = G mearth mspacecraft r2 while that exerted on the spacecraft by the moon is Fmoon = G mmoon mspacecraft ( rearth-moon r ) 2 By setting these two expressions equal to each other (since the gravitational forces balance), we will be able to find the distance r. Point where the gravitational forces b Earth Moon SOLUTION Setting Fearth equal to Fmoon, we have G mearth mspacecraft Solving this expression for r gives r2 =G mmoon mspacecraft ( rearth-moon r ) 2 238 FORCES AND NEWTON'S LAWS OF MOTION mearth mmoon ( rearth-moon ) r= mearth mmoon 1+ = ( 3.85 108 m ) 81.4 1 + 81.4 = 3.47 108 m _____________________________________________________________________________ _ 115. SSM REASONING AND SOLUTION The free-body diagram is shown at the right. The forces that act on the picture are the pressing force P, the normal force FN exerted on the picture by the wall, the weight mg of the picture, and the force of static friction fsMAX . The maximum magnitude for MAX fS FN P mg fsMAX = s FN . The picture is in the frictional force is given by Equation 4.7: equilibrium, and, if we take the directions to the right and up as positive, we have in the x direction Fx = P FN = 0 or P = FN Fy = fsMAX mg = 0 or fsMAX = mg and in the y direction Therefore, fsMAX = s FN = mg But since FN = P , we have s P = mg Solving for P, we have P= mg (1.10 kg)(9.80 m/s 2 ) = = 16.3 N s 0.660 ____________________________________________________________________________________________ 116. REASONING AND SOLUTION a. The rope exerts a tension, T, acting upward on each block. Applying Newton's second law to the lighter block (block 1) gives T m1g = m1a Similarly, for the heavier block (block 2) T m2g = m2a Chapter 4 Problems 239 Subtracting the second equation from the first and rearranging yields m m1 a= 2 g = 3.68 m/s 2 m +m 1 2 b. The tension in the rope is now 908 N since the tension is the reaction to the applied force exerted by the hand. Newton's second law applied to the block is T m1g = m1a Solving for a gives a= ( 908 N ) 9.80 m/s 2 = 11.8 m/s 2 T g= m1 42.0 kg c. In the first case, the inertia of BOTH blocks affects the acceleration whereas, in the second case, only the lighter block's inertia remains. ____________________________________________________________________________________________ 117. SSM REASONING AND SOLUTION The penguin comes to a halt on the horizontal surface because the kinetic frictional force opposes the motion and causes it to slow down. The time required for the penguin to slide to a halt ( v = 0 m/s) after entering the horizontal patch of ice is, according to Equation 2.4, t= v v0 ax = v0 ax We must, therefore, determine the acceleration of the penguin as it slides along the horizontal patch (see the following drawing). fk1 FN1 mg sin mg cos Free-body diagram A fk2 FN2 mg Free-body diagram B For the penguin sliding on the horizontal patch of ice, we find from free-body diagram B and Newton's second law in the x direction (motion to the right is taken as positive) that 240 FORCES AND NEWTON'S LAWS OF MOTION Fx = f k2 = ma x or ax = f k2 k FN2 = m m In the y direction in free-body diagram B, we have Fy = FN2 mg = 0 , or FN2 = mg . Therefore, the acceleration of the penguin is ax = k mg = k g m (1) Equation (1) indicates that, in order to find the acceleration ax, we must find the coefficient of kinetic friction. We are told in the problem statement that the coefficient of kinetic friction between the penguin and the ice is the same for the incline as for the horizontal patch. Therefore, we can use the motion of the penguin on the incline to determine the coefficient of friction and use it in Equation (1). For the penguin sliding down the incline, we find from free-body diagram A (see the previous drawing) and Newton's second law (taking the direction of motion as positive) that Fx = mg sin f k1 = ma x = 0 f k1 = mg sin or (2) Here, we have used the fact that the penguin slides down the incline with a constant velocity, so that it has zero acceleration. From Equation 4.8, we know that f k1 = k FN1 . Applying Newton's second law in the direction perpendicular to the incline, we have Fy = FN1 mg cos = 0 or FN1 = mg cos Therefore, f k1 = k mg cos , so that according to Equation (2), we find f k1 = k mg cos = mg sin Solving for the coefficient of kinetic friction, we have k = sin = tan cos Finally, the time required for the penguin to slide to a halt after entering the horizontal patch of ice is v v0 v0 1.4 m/s t= 0 = = = = 1.2 s ax k g g tan (9.80 m/s 2 ) tan 6.9 Chapter 4 Problems 241 ____________________________________________________________________________________________ 118. REASONING The following figure shows the crate on the incline and the free body diagram for the crate. The diagram at the far right shows all the forces resolved into components that are parallel and perpendicular to the surface of the incline. We can analyze the motion of the crate using Newton's second law. The coefficient of friction can be determined from the resulting equations. y FN x P sin MAX P MAX FN fS fS P P cos mg sin mg mg cos SOLUTION Since the crate is at rest, it is in equilibrium and its acceleration is zero in all directions. If we take the direction down the incline as positive, Newton's second law indicates that Fx = P cos + mg sin f sMAX = 0 According to Equation 4.7, fsMAX = s FN . Therefore, we have P cos + mg sin s FN = 0 (1) The expression for the normal force can be found from analyzing the forces that are perpendicular to the incline. Taking up to be positive, we have Fy = P sin + FN mg cos = 0 or FN = mg cos P sin Equation (1) then becomes P cos + mg sin s (mg cos P sin ) = 0 Solving for the coefficient of static friction, we find that s = P cos + mg sin (535 N) cos 20.0 + (225 kg)(9.80 m/s 2 ) sin 20.0 = = 0.665 mg cos P sin (225 kg)(9.80 m/s 2 ) cos 20.0 (535 N) sin 20.0 ____________________________________________________________________________________________ 242 FORCES AND NEWTON'S LAWS OF MOTION 119. REASONING The free-body diagram for the box is shown in the following drawing on the left. On the right the same drawing is repeated, except that the pushing force P is resolved into its horizontal and vertical components. P P sin FN FN P cos fk fk mg mg Since the block is moving at a constant velocity, it has no acceleration, and Newtons second law indicates that the net vertical and net horizontal forces must separately be zero. SOLUTION Taking upward and to the right as the positive directions, we write the zero net vertical and horizontal forces as follows: F mg P sin = 0 1N4 4 4 2 4 4 4 3 Vertical P cos f = 0 1 4 4 2 4 k43 Horizontal From the equation for the horizontal forces, we have P cos = fk . But the kinetic frictional force is fk = kFN . Furthermore, from the equation for the vertical forces, we have FN = mg + P sin . With these substitutions, we obtain P cos = f k = k FN = k ( mg + P sin ) Solving for P gives P= k mg cos k sin The necessary pushing force becomes infinitely large when the denominator in this expression is zero. Hence, we find that cos k sin = 0 , which can be rearranged to show that sin 1 1 = tan = or = tan 1 = 68 cos k 0.41 ____________________________________________________________________________________________
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