Chapter 4 - CHAPTER 4 Newton's Laws Note: For all problems...

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CHAPTER 4 Newton’s Laws Note: For all problems we shall take the upward direction as positive unless otherwise stated. 1* ·· How can you tell if a particular reference frame is an inertial reference frame? If Newton’s first law is obeyed, the reference frame is an inertial reference frame. 2 ·· Suppose you find that an object in a particular frame has an acceleration a when there are no forces acting on it. How can you use this information to find an inertial reference frame? If the object has an acceleration a in the absence of external forces, its reference frame has an acceleration - a . An inertial reference frame is then one that accelerates with acceleration - a relative to the reference frame in which the object has the acceleration a in the absence of external forces. 3 · If an object has no acceleration in an inertial reference frame, can you conclude that no forces are acting on it? No; one can only conlcude that the net force is zero. 4 · If only a single force acts on an object, must the object accelerate in an inertial reference frame? Can it ever have zero velocity? Yes, it must accelerate. It can have zero velocity at some instant. 5* · If an object is acted upon by a single known force, can you tell in which direction the object will move using no other information? No; only its acceleration is known (assuming one knows the mass). 6 · An object is observed to be moving at constant velocity in an inertial reference frame. It follows that ( a ) no forces act on the object. ( b ) a constant force acts on the object in the direction of motion. ( c ) the net force acting on the object is zero. ( d ) the net force acting on the object is equal and opposite to its weight. ( c ) See Problem 4-3. 7 · A body moves with constant speed in a straight line in an inertial reference frame. Which of the following statements must be true? ( a ) No force acts on the body. ( b ) A single constant force acts on the body in the
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Chapter 4 Newton’s Laws direction of motion. ( c ) A single constant force acts on the body in the direction opposite to the motion. ( d ) A net force of zero acts on the body. ( e ) A constant net force acts on the body in the direction of motion. ( d ) a = 0, therefore F net = 0. 8 ·· Figure 4-23 shows the position x versus time t of a particle moving in one dimension. During what time intervals is there a net force acting on the particle? Give the direction (+ or -) of the net force during these time intervals. Between t = 0 and t = 2 s, and between t = 8 s and t = 10 s, the velocity is constant and a = 0 and F = 0. Between t = 2 s and t = 5 s, a is negative and F is negative; between t = 5 s and t = 8 s, a is positive and so is F . 9* · A particle of mass m is traveling at an initial speed v 0 = 25.0 m/s. It is brought to rest in a distance of 62.5 m when a net force of 15.0 N acts on it. What is m ? ( a ) 37.5 kg ( b ) 3.00 kg ( c ) 1.50 kg ( d ) 6.00 kg ( e ) 3.75 kg a = v 2 /2 s = F / m ; m = v 2 /2 Fs m = 3.00 kg; ( b 10 · ( a ) An object experiences an acceleration of 3 m/s 2 when a certain force F 0 acts on it. What is its acceleration when the force is doubled? ( b
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Chapter 4 - CHAPTER 4 Newton's Laws Note: For all problems...

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