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Assignment Homework 3 Solutions Problem 2.33: Air-bag injuries. During an auto accident, the vehicle's air bags deploy and slow down the passengers more gently than if they had hit the windshield or steering wheel. According to safety standards, the bags produce a maximum acceleration of 60 g, but lasting for only 36 ms (or less). How far (in meters) does a person travel in coming to a complete stop in 36 ms at a constant acceleration of 60g? Answer: 38.1 cm Before we try to solve this problem, convert 36 ms into seconds: 1 sec 36 ms =0.036 sec 1000 ms Then we need to use two kinematic equations. First we use v =v o at to find the initial velocity v o (or velocity before the braking started). We assume v = 0 (because it stops): v o=at v o =609.8 m / s 20.036 s v o= 21.2 m / s Note that the acceleration is negative (i.e., a deceleration). Then, we use the second kinematic equation, 2 2 v =v o 2a x , to find the distance x traveled in 0.036 sec (remember v = 0 because it stops): v x= o 2a 21.2 m / s 2 x= 2 60 9.8 m / s 2 x = 0.381 m x =38.1 cm 2 Problem 2.38 The "reaction time" of the average automobile driver is about 0.700 s. (The reaction time is the interval between the perception of a signal to stop and the application of the brakes.) If an automobile can slow down with an acceleration of 12.0 ft / s 2 , compute the total distance covered in coming to a stop after a signal is observed (a) from an initial velocity of 15.0 mi/h (in a school zone) and (b) from an initial velocity of 55.0 mi/h. A) Answer: 10.8 m First, convert 12.0 ft / s 2 into m/s: 12 ft 1 meter 2 =3.7 m / s 2 1 s 3.28 ft And convert 15.0 mi/h into m/s: 15.0 mi 1609 meters 1 hr = 6.7 m / s 1 hr 1 mi 3600 sec Then to solve, we need to break the problem into two parts. The first part is when the car is not braking due to the fact that the driver has a reaction time. This distance d 1 , is easy to calculate: d 1= v o t d 1=6.7 m / s 0.7 s d 1= 4.69 m Then we need to use the kinematic equation v =v o 2a x to find the second distance d 2 . In this equation, x is our d 2 and v = 0 . v 2 o 2a 6.7m / s 2 d 2= 2 3.7 m / s 2 d 2=6.1 m d 2= Finally, we get d total by adding up d 1 and d 2 : d total =d 1d 2 d total =10.8 m 2 2 B) 99.9 m First convert 55.0 m/h into m/s: 55.0 mi 1609 meters 1 hr =24.6 m / s 1 hr 1 mi 3600 sec Then we solve it the same way that we solved A: d 1= v o t d 1= 24.6 m / s 0.7 s d 1=17.2 m v o 2a 24.6 m / s 2 d 2= 2 3.7 m / s 2 d 2=81.8 m d 2= d total = d 1 d 2 d total =99.9 m 2 Problem 2.73 It has been suggested, and not facetiously, that life might have originated on Mars and been carried to Earth when a meteor hit Mars and blasted pieces of rock (perhaps containing primitive life) free of the surface. Astronomers know that many Martian rocks have come to Earth this way. (For information on one of these, search the Internet for ALH 84001.) One objection to this idea is that microbes would have to undergo an enormous, lethal acceleration during the impact. Let us investigate how large such an acceleration might be. To escape Mars, rock fragments would have to reach its escape velocity of 5.0 km/s, and this would most likely happen over a distance of about 4.0 m during the impact. A) What would be the acceleration, in m/s2, of such a rock fragment? Answer: 3.1310 6 m / s2 First convert, 5.0 km/s into m/s: 5.0 km 1000 m =5000 m / s 1 sec 1 km To find the acceleration, use: v =v o 2a x Where v =0 m / s , v o=5000 m / s , and x = 4.0 m : v2 o 2 x 5000 m / s 2 a= 2 4.0 m a =3.1310 6 m / s 2 a= B) What would be the acceleration, in g's, of such a rock fragment? Answer: 3.1910 5 g ' s Just divide the answer in A by 9.8 m / s 2 : 3.1310 m / s =3.19 105 g ' s 2 9.8 m / s 6 2 2 2 C) How long would this acceleration last? Answer: 1.60 ms Use the following kinematic: Set v = 0 and solve for t (and convert to ms): v o a 5000 m / s t= 3.13 106 m / s 2 t =1.60 103 s t =1.60 ms t= D) In tests, scientists have found that over 40% of Bacillius subtilis bacteria survived after an acceleration of 450000g. In light of your answer to part A, can we rule out the hypothesis that life might have been blasted from Mars to Earth? Answer: No. v =v o at Problem 2.72: Prevention of hip fractures. Falls resulting in hip fractures are a major cause of injury and even death to the elderly. Typically, the hips speed at impact is about 2.0 m/s. If this can be reduced to 1.3 m/s or less, the hip will usually not fracture. One way to do this is by wearing elastic hip pads. A) If a typical pad is 5.0 cm thick and compresses by 2.0 cm during the impact of a fall, what acceleration (in m/s2) does the hip undergo to reduce its speed to 1.3 m/s? Answer: 57.8 m / s 2 First convert everything to m: 5 cm=0.05 m , 2.0 cm =0.02 m Now, use the following kinematic equation and solve for a: v =v o 2a x 2 2 Here, v o= 2.0 m / s , v =1.3 m / s and x =0.02 m : v 2 v 2 o 2 x 1.3 m / s 2 2.0 m / s 2 a= 2 0.02m a=57.8 m / s 2 a= B) What acceleration (in g's ) does the hip undergo to reduce its speed to 1.3 m/s? Answer: 5.89 g's Just divide A by 9.8 m / s 2 : a= 57.8 m / s 2 2 9.8 m/ s a=5.89 g ' s C) The acceleration you found in part A may seem like a rather large acceleration, but to fully assess its effects on the hip, calculate how long it lasts. Answer: 1.21102 sec Just use: and solve for t: v = v oat v v 0 a 2.0 m/ s 1.3 m / s t= 57.8 m / s 2 t =1.21 102 sec t= Problem 2.58: Physiological effects of large acceleration. The rocketdriven sled Sonic Wind No. 2, used for investigating the physiological effects of large accelerations, runs on a straight, level track that is 1080 m long. Starting rest, from it can reach a speed of 1610 km/h in 1.80 s. A) Compute the acceleration in m/s2. Answer: 248 m / s2 First, convert the speed into m/s: 1610 km 1 hour 1000 m = 447.2 m/ s 1 hour 3600 sec 1 km Now use the equation: v =v o at Where v = 447.2 m / s , v o= 0 m / s , and t =1.8 s . Solve for a: v vo t 447.2 m / s a= 1.8 s a = 248 m / s 2 a= B) Compute the acceleration in gs. Answer: 25.3 g's Again, divide A by 9.8 m / s 2 : a= 248 m / s 2 9.8 m/ s 2 a= 25.3 g ' s C) What is the distance covered in 1.80 s? Answer: 403 m To find x use: 12 x = x o v o t a t 2 And set v o =0 m / s , t =1.8 s , and a =248 m / s 2 : 1 2 2 x = 248 m / s 1.8 s 2 x =403 m D) A magazine article states that, at the end of a certain run, the speed of the sled decreased from 1020 km/h to zero in 1.40 s and that, during this time, its passenger was subjected to more than 40g. Are these figures consistent? Answer: No. Tossing Balls off a Cliff Learning Goal: To clarify the distinction between speed and velocity, and to review qualitatively onedimensional kinematics.A woman stands at the edge of a cliff, holding one ball in each hand. At time t 0 , she throws one ball straight up with speed v 0 and the other straight down, also with speed v 0 . For the following questions neglect air resistance. Pay particular attention to whether the answer involves absolute quantities that have only magnitude (e.g., speed) or quantities that can have either sign (e.g., velocity). Take upward to be the positive direction. A) If the ball that is thrown downward has an acceleration of magnitude a at the instant of its release (i.e., when there is no longer any force on the ball due to the woman's hand), what is the relationship between a and g, the magnitude of the acceleration of gravity? a) a > g b) a = g c) a < g Register to View AnswerThe only acceleration that is acting on the ball once it is released is an acceleration due to gravity. B) Which ball has the greater acceleration at the instant of release? a) the ball thrown upward b) the ball thrown downward c) Neither; the accelerations of both balls are the same. Register to View AnswerAgain, the only acceleration acting on either ball is that due to gravity. They both feel the same amount of acceleration due to gravity. C) Which ball has the greater speed at the instant of release? a) the ball thrown upward b) the ball thrown downward c) Neither; the speeds are the same. Register to View AnswerThough the ball thrown upward has an upward, positive velocity and the ball thrown downward has a downward, negative velocity, their speeds (or magnitudes of velocity) are the same. D) Which ball has the greater average speed during the 1-s interval after release (assuming neither hits the ground during that time)? a) the ball thrown upward b) the ball thrown downward c) Neither; the speeds are the same. Register to View AnswerThe ball thrown down will accelerate while the ball thrown upward will first slow down. E) Which ball hits the ground with greater speed? a) the ball thrown upward b) the ball thrown downward c) Neither; the balls hit the ground with the same speed. Register to View AnswerEven though in D, the ball thrown downward has a larger velocity 1-s after release, they both hit the ground with the same speed. However, the ball thrown upward will hit the ground after the ball thrown downward. Rocket Height A rocket, initially at rest on the ground, accelerates straight upward from rest with constant acceleration 39.2 m / s 2 . The acceleration period lasts for time 6.00 s until the fuel is exhausted. After that, the rocket is in free fall. A) Find the maximum height y max reached by the rocket. Ignore air resistance and assume a constant acceleration due to gravity equal to 9.8 m/s2. Answer: 3530 m This problem has two stages. For the first stage, which lasts 6.00 s, the rocket experiences an acceleration of: a 1=39.2 m / s 2 In the second stage, the acceleration is: a 2=9.8 m / s 2 The total height will be a sum of the distance traveled in either stage: h =h 1 h2 So now, to find h1 : 12 h 1= v o t a 1 t 2 where v o= 0 m / s , a1=39.2 m / s2 , and t =6.00 s : 1 h 1= 39.2 m / s 2 6.00 s 2 2 h1=706 m And to find h 2 , we use the kinematic equation: v =v o 2a x where x = h2 , a =a 2 , v = 0 m / s (at final height), and v_o is the final velocity from first stage. To find v o : v =v o at v = 39.2 m / s 2 6.00 s v =235.2 m / s (from first stage) 2 2 Now we are ready to find h 2 : 2 v o 2 a2 235.2 m / s 2 h 2= 2 9.8 m / s 2 h2= 2824 m h 2= So now, to find h, just add up h1 and h 2 : h= h1h 2 h =706 m2824 m h= 3530 m Problem 2.57 A rock is thrown vertically upward with a speed of 12.0 m/s from the roof of a building that is 60.0 m above the ground. Assume free fall. A) In how many seconds after being thrown does the rock strike the ground? Answer: 4.93 s Draw a picture: vo 60.0 m To solve this problem efficiently, we should use the equation: 12 x = xo v o t a t 2 Here, x x o=60.0 m , v o=12 m/ s and a =9.8 m/ s 2 . We know everything except t. This means that to solve this equation, we'll need to solve a quadratic. Before we plug numbers in, lets put it into the standard form for the quadratic formula: 12 a t v o t x x o= 0 2 From this, we can plug it into our quadratic formula: 1 v o v 2 4 a x x o o 2 t= 1 2 a 2 2 12 m / s 12 m / s 2 9.8 m / s 2 60.0m t= 9.8 m/ s 2 t =1.22 s 3.71 s t =4.93 s or 2.49 s We can see from the calculations that the only possible answer is 4.93 s. B) What is the speed of the rock just before it strikes the ground? Answer: 36.3 m/s To answer this, we can use: v 2 = v 2 2 a x o 2 v = vo 2 a x v = 12 m / s 22 9.8 m / s2 60.0 m v =36.3 m / s ... View Full Document

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