Chapter 5: Newton’s Laws of MotionAnswers to Even-Numbered Conceptual Questions2. If the tablecloth is pulled rapidly, it can exert a force on the place settings for only a very short time. In thisbrief time, the objects on the table accelerate, but only slightly. Therefore, the objects may have barely movedby the time the tablecloth is completely removed.4. As the dog shakes its body, it starts water in its fur moving in one direction. When it then begins to shake itsbody in the opposite direction, much of the water continues in the same direction because of the law ofinertia (Newton’s First Law). As a result, water leaves the fur with each reversal in direction.6. The drag racer needs an engine to turn the wheels, which makes them push against the ground. It is only inthis way that the ground is able to exert a reaction force on the car.8. The astronaut should push the jet pack away from him, in the opposite direction from the spaceship. As aresult, the reaction force exerted on him by the pack will accelerate him toward the ship.10. The action-reaction forces are the force exerted on the ball by the bat, and the force exerted on the bat by theball. The force exerted on the ball causes it to change its direction of motion and move into the field of play(hopefully). The force exerted on the bat slows its forward motion.12. The whole brick also experiences twice the force due to gravity. As a result, these two effects (more inertia,more force) exactly cancel, and the free fall acceleration is independent of mass.14. No. If only a single force acts on the object, it will not stay at rest; instead, it will accelerate in the direction ofthe force.16. (a) When the parrot jumps, it pushes down on its perch. In this case, the scale reads a larger value. (b) Whenthe parrot is in the air, the scale reads only the weight of the bird cage. (c) When the parrot lands, the perchexerts an upward force on it to bring it to rest. As a result, the scale again reads a larger value. The average ofthe scale readings in parts (a), (b), and (c) is just the weight of the parrot plus the weight of the bird cage.18. Since the car is at rest, we can conclude that it has zero net force acting on it. This is not the same as havingno forces at all acting on the car. In fact, gravity exerts a downward force, and the road exerts an equal andopposite net upward force (summed over the four tires).20. As you hit the rug with the tennis racket, you cause it to accelerate rapidly. The dust on the rug, if it is notattached too firmly, will be left behind as the rug accelerates away from it. In this way, you are able toremove much of the dust from the rug.22. Consider a projectile that moves with no air resistance. (a) If the projectile is released from rest it will movein the same direction as the net force acting on it. (b) On the other hand, suppose the projectile is throwninto the air at an angle to the horizontal. At the top of its flight it moves horizontally while the net forceacting on it (gravity) acts in the vertical direction, at right angles to the direction of motion. (c) If a projectileis thrown straight upward, its initial motion is in the opposite direction of the net force acting on it.24. Yes, it would still hurt. The reason is that even though the bucket is “weightless,” it still has nonzero mass.Therefore, it has inertia, and when it is kicked it will resist having its state of motion changed. The way itresists a change in its motion is by exerting an equal-but-opposite reaction force on the object trying to makethe change; in this case, your foot. This is the force that can make kicking the bucket painful.26. In everyday units, 1 N is approximately a quarter pound. Objects with a weight of roughly a quarter poundinclude a quarter-pound hamburger, an apple, a glass of water, and half a dozen Fig Newtons.Copyright © 2010 Pearson Education, Inc. All rights reserved. This material is protected under all copyright laws as they currently exist. Noportion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.5 – 1Chapter 5: Newton’s Laws of Motion James S. Walker, Physics, 4th EditionCopyright © 2010 Pearson Education, Inc. All rights reserved. This material is protected under all copyright laws as they currently exist. Noportion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.5 – 2Chapter 5: Newton’s Laws of Motion James S. Walker, Physics, 4th EditionSolutions to Problems and Conceptual Exercises1. Picture the Problem: A force of magnitude F acts on an object of mass m that is initially at rest and accelerates it to speed v in a time T.Strategy: Use the concepts of Newton’s Second Law and the definition of acceleration to construct a ratio that will helpus predict the effect of changing m and F on the time interval T.Solution: 1. Construct a ratio: TnewTold=v−v0( )anewv−v0( )aold=aoldanew=FoldmoldFnewmnew=FoldFnewmnewmold=F4F2mm=122. Solve for Tnew in terms of T: Tnew=12Told=12TInsight: If the mass m were quadrupled instead of doubled, the acceleration would be the same and so would be the time required to accelerate to speed v.2. Picture the Problem: The free body diagram for this problem is shown at right.Strategy: Use the free body diagram to determine the net force on the rock, then apply Newton’s Second Law to find the acceleration of the rock. Let upward be the positive direction.Solution: 1. Find the net force: rF∑= −40.0 N( )ˆy+ 46.2 N( )ˆy= 6.2 N( )ˆy2. Now apply Newton’s Second Law (equation 5-1) to find ra: ra =rF∑m=6.2 N( )ˆy5.00 kg= 1.2 m/s2( )ˆyInsight: If the astronaut were to exert less than 40.0 N of upward force on the rock, it would accelerate downward.3. Picture the Problem: A shopping cart accelerates in the forward direction due to a pushing force.Strategy: Use Newton’s Second Law to find the acceleration of the cart, then use the acceleration to find the distance traveled in 2.50 s.Solution: 1. Find ra using equation 5-1: ra =rFm=10.1 N( )ˆx12.3 kg= 0.821 m/s2( )ˆx2. Use equation 2-11 to find the distance traveled: Δx =12at2=120.821 m/s2( )2.50 s( )2= 2.57 mInsight: Doubling the applied force would double the acceleration and double the distance traveled.4. Picture the Problem: You exert a horizontal force that accelerates both your little sister and the sled.Strategy: Apply