Cap. 5 Solucionario Serway

5
The Laws of Motion
CHAPTER OUTLINE
5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 The Concept of Force Newton’s First Law and Inertial Frames Mass Newton’s Second Law The Gravitational Force and Weight Newton’s Third Law Some Applications of Newton’s Laws Forces of Friction

ANSWERS TO QUESTIONS
Q5.1 (a) The force due to gravity of the earth pulling down on the ball—the reaction force is the force dueto gravity of the ball pulling up on the earth. The force of the hand pushing up on the ball—reaction force is ball pushing down on the hand. (b) The only force acting on the ball in free-fall is the gravity due to the earth—the reaction force is the gravity due to the ball pulling on the earth. Q5.2 *Q5.3 The resultant force is zero, as the acceleration is zero. Answer (b). An air track or airtable is a wonderful thing. It exactly cancels out the force of the Earth’s gravity on the gliding object, to display free motion and to imitate the effect of being far away in space.

Q5.4

When the bus starts moving, the mass of Claudette is accelerated by the force of the back of the seat on her body. Clark is standing, however, and the only force on him is the friction between his shoes andthe floor of the bus. Thus, when the bus starts moving, his feet start accelerating forward, but the rest of his body experiences almost no accelerating force (only that due to his being attached to his accelerating feet!). As a consequence, his body tends to stay almost at rest, according to Newton’s first law, relative to the ground. Relative to Claudette, however, he is moving toward her and fallsinto her lap. Both performers won Academy Awards. Shake your hands. In particular, move one hand down fast and then stop your hand abruptly. The water drops keep moving down, according to Newton’s first law, and leave your hand. This method is particularly effective for fat, large-mass, high-inertia water drops. First ask, “Was the bus moving forward or backing up?” If it was moving forward, thepassenger is lying. A fast stop would make the suitcase fly toward the front of the bus, not toward the rear. If the bus was backing up at any reasonable speed, a sudden stop could not make a suitcase f ly far. Fine her for malicious litigiousness. (a) The air inside pushes outward on each patch of rubber, exerting a force perpendicular to that section of area. The air outside pushes perpendicularlyinward, but not quite so strongly. (b) As the balloon takes off, all of the sections of rubber feel essentially the same outward forces as before, but the now-open hole at the opening on the west side feels no force. The vector sum of the forces on the rubber is to the east. The small-mass balloon moves east with a large acceleration. (c) Hot combustion products in the combustion chamber pushoutward on all the walls of the chamber, but there is nothing for them to push on at the open rocket nozzle. The net force exerted by the gases on the chamber is up if the nozzle is pointing down. This force is larger than the gravitational force on the rocket body, and makes it accelerate upward.
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*Q5.5

Q5.6

*Q5.7

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Chapter 5*Q5.8

A portion of each leaf of grass extends above the metal bar. This portion must accelerate in order for the leaf to bend out of the way. The leaf’s mass is small, but when its acceleration is very large, the force exerted by the bar on the leaf puts the leaf under tension large enough to shear it off. The molecules of the floor resist the ball on impact and push the ball back, upward.The actual force acting is due to the forces between molecules that allow the floor to keep its integrity and to prevent the ball from passing through. Notice that for a ball passing through a window, the molecular forces weren’t strong enough.

Q5.9

*Q5.10 The child exerts an upward force on the ball while it is in her hand, but the question is about the ball moving up after it leaves her...