# Chap4 - CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1(b If only one force acts on the object it is the net

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CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (b) If only one force acts on the object, it is the net force; thus, the net force must be nonzero. Consequently, the velocity would change, according to Newton’s first law, and could not be constant. 3. (e) Newton’s first law states that an object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. All three statements are consistent with the first law. 5. (c) Newton’s second law gives the answer directly, provided the net force is calculated by vector addition of the two given forces. The direction of the net force gives the direction of the acceleration. 7. (e) Answers a and b are false, according to the third law, which states that whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. It does not matter whether one of the bodies is stationary or whether it collapses. Answer c is false, because according to the third law, Sam and his sister experience forces of equal magnitudes during the push-off. Since Sam has the greater mass, he flies off with the smaller acceleration, according to the second law. Answer d is false, because in catching and throwing the ball each astronaut applies a force to it, and, according to the third law, the ball applies an oppositely directed force of equal magnitude to each astronaut. These reaction forces accelerate the astronauts away from each other, so that the distance between them increases. 8. (b) Newton’s third law indicates that Paul and Tom apply forces of equal magnitude to each other. According to Newton’s second law, the magnitude of each of these forces is the mass times the magnitude of the acceleration. Thus, we have m Paul a Paul = m Tom a Tom , or m Paul / m Tom = a Tom / a Paul . 11. (a) The answer follows directly from the fact that weight W is given by W = mg , where m is the mass and g is the acceleration due to the earth’s gravity. Thus, m = (784 N)/(9.80 m/s 2 ) = 80.0 kg. The mass is the same on Mercury as on Earth, because mass is an intrinsic property of matter. 12. (d) What matters is the direction of the elevator’s acceleration. When the acceleration is upward, the apparent weight is greater than the true weight. When the acceleration is downward, the apparent weight is less than the true weight. In both possibilities the acceleration points upward.

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160 FORCES AND NEWTON'S LAWS OF MOTION 13. (b) According to Newton’s third law, the pusher and the wall exert forces of equal magnitude but opposite directions on each other. The normal force is the component of the wall’s force that is perpendicular to the wall. Thus, it has the same magnitude as the component of the pusher’s force that is perpendicular to the wall. As a result, the normal forces are ranked in the same order as the perpendicular components of the pusher’s forces.
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## This note was uploaded on 10/23/2011 for the course BIO 201 taught by Professor Frenkel/cervantes during the Spring '10 term at Rutgers.

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Chap4 - CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1(b If only one force acts on the object it is the net

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