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. 2. (d) This situation violates the first law, which predicts that the rabbit’s foot tends to remain in place where it was when the car begins accelerating. The car would leave the rabbit’s foot behind. Th at is, the rabbit’s foot would swing away from, not toward, the windshield. 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. 4. (a) Newton’s second law with a net force of 7560 N – 7340 N = 220 N due north gives the answer directly. 5. (c) Newton’s second law gives the answer directly, provided t he net force is calculated by vector addition of the two given forces. The direction of the net force gives the direction of the acceleration. 6. (e) Newton’s second law gives the answer directly. One method is to determine the total acceleration by vector addition of the two given components. The net force has the same direction as 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 .
Chapter 4 Answers to Focus on Concepts Questions 159 9. (e) Newton’s law of gravitation gives the answer directly. According to this law the weight is directly proportional to the mass of the planet, so twice the mass means twice the weight. However, this law also indicates that the weight is inversely proportional to the square of the planet’s radius, so three times the radius means one ninth the weight. Together, these two factors mean that the weight on the planet is 2/9 or 0.222 times your earth-weight.
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