Chapter 6: Applications of Newton’s LawsAnswers to Even-Numbered Conceptual Questions2. Spinning the wheels is likely to decrease the force exerted by the Jeep. The reason is that the force exerted bythe spinning wheels is kinetic friction, and the coefficient of kinetic friction is generally less than thecoefficient of static friction. The spinning wheels look more exciting in the movie, however.4. The maximum acceleration is determined by the normal force exerted on the drive wheels. If the engine ofthe car is in the front, and the drive wheels are in the rear, the normal force is less than it would be withfront-wheel drive. During braking, however, all four wheels participate—including the wheels that sit underthe engine.6. Friction is beneficial whenever you want to start, stop, or turn a car. It is also beneficial when you tune aguitar, tie your shoes, or sprint from rest to catch a bus.8. This is possible because if you spin the bucket rapidly enough, the force needed to produce circular motionis greater than the force of gravity. In this case, a force in addition to gravity must act at the top of the circleto keep the water moving in its circular path. This force is provided by the bottom of the bucket. Therefore,the bottom of the bucket pushes against the water, and the water pushes back against the bucket—this keepsthe water from falling out of the bucket.10. Yes. Equilibrium simply means that the net force acting on an object is zero. Therefore, an object movingwith constant velocity can be considered to be in equilibrium. In a frame of reference moving with the samevelocity, the object would be at rest and would have zero net force acting on it—which is the way we usuallythink of equilibrium.12. Astronauts feel weightless because they are in constant free fall as they orbit, just as you would feelweightless inside an elevator that drops downward in free fall.14. A passenger on this ride feels pushed against the wall, which means that the wall, in turn, exerts a normalforce on the passenger. If the corresponding force of static friction is greater than the passenger’s weight, thepassenger stays put as the floor is lowered away. (Why do all passengers stay put at the same rotationalspeed, regardless of their weight?)16. If the parking brake is applied while the car is in motion, the rear wheels begin to skid across the pavement.This means that the friction acting on the rear wheels is kinetic friction, which is smaller in magnitude thanthe static friction experienced by the front wheels. As a result, the rear wheels will overtake the front wheels,causing the car to spin around and begin moving rear wheels first. This is standard procedure for stuntdrivers wishing to spin a car around in a chase scene.18. As the basket within a washing machine rotates, the clothes collect on the rim of the basket. Here the basketexerts an inward force on the clothes, causing them to follow a circular path. The water contained in theclothes, however, is able to pass through the holes of the basket where it can be drained from the machine.20. People on the outer rim of a rotating space station must experience a force directed toward the center of thestation in order to follow a circular path. This force is applied by the “floor” of the station, which is really itsoutermost wall. Because people feel an upward force acting on them from the floor, just as they would onEarth, the sensation is like an “artificial gravity.”22. The physics of this scene is somewhere between “bad” and “ugly.” When the rope burns through, Robin ismoving horizontally. This horizontal motion should continue as Robin falls, leading to a parabolic trajectoryrather than the straight downward drop shown in the movie.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.6 – 1Chapter 6: Applications of Newton’s Laws 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.6 – 2Chapter 6: Applications of Newton’s Laws James S. Walker, Physics, 4th EditionSolutions to Problems and Conceptual Exercises1. Picture the Problem: Two bricks are pushed across a rough surface. In case 1 they are placed end-to-end and in case 2 they are stacked one on top of the other.Strategy: Use the mathematical relationship between the force of kinetic friction and the normal force to answer the conceptual question.Solution: 1. (a) The force of kinetic friction is proportional to the normal force between the bricks and the tabletop. In case 2 the normal force will be twice the normal force of a single brick, but in case 1 there are two bricks and therefore two friction forces. Another way to view the situation is to say there are two bricks of the same mass but different shapes. However, the friction force does not depend upon shape, only upon the magnitude of the normal force. We conclude that the force of kinetic friction in case 1 is equal to the force of kinetic friction in case 2.2. (b) The best explanation is II. The normal force is the same in the two cases, and friction is independent of surface area. Statements I and III are both false.Insight: If we were to stand the bricks up on their small ends, the friction forces would still remain the same.2. Picture the Problem: Two drivers stop suddenly to avoid a deer. Driver 1 stops by locking up his brakes and screeching to a halt; driver 2 stops by applying her brakes just to the verge of locking, so that the wheels continue to turnuntil her car comes to a complete stop.Strategy: Use Newton’s Second Law together with the difference between kinetic and static friction to answer the conceptual question.Solution: 1. (a) Driver 1 stops his car using kinetic friction as the car tires skid, but driver 2 uses static friction because the point of contact between a rolling tire and the road is instantaneously at rest. The maximum magnitude of the static friction force is larger than the magnitude of the kinetic friction force, so driver 2 has a larger braking