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Transcript
Section Review Answers Chapter 12 Section 1 1. Answers may vary. Students should say in their own words that an object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force. They should then give an example of this law. 2. Answers may vary. Sample answer: a student pushing a piano across a floor, an apple falling from a tree 3. Answers may vary. Sample answer: When you have on a seatbelt and the vehicle stops suddenly, the seatbelt applies a force that stops you and keeps you from continuing forward, as inertia keeps you in motion after the car has stopped. 4. a. Newton’s second law b. Newton’s first law c. Newton’s first law 5. a. The car may be unable to turn. Newton’s first law states that the object (car) will continue to travel in a straight line unless an unbalanced force acts on the object. Since the road is icy, the friction between the tires and the ice may not be large enough to turn the car. b. The car will slide for the same reasons in (a). Also, Newton’s second law states that acceleration is proportional to force. Since the friction force is much smaller on an icy road, the negative acceleration (“deceleration”) is much smaller. 6. a = F/m = 15 N/58 kg = 0.26 m/s2 forward 7. m = F/a = 34 N/4 m/s2 = 8.5 kg Section 2 1. The moon’s mass is much smaller than Earth’s mass. Since the force of gravitational attraction between two objects depends on the mass of both objects, the force of gravitational attraction between you and the moon would therefore be smaller than the force between you and Earth. 2. Sample answer: The law of universal gravitation says that the force of gravitational attraction is proportional to the attracting masses and inversely proportional to the square of the distance between the masses. If you make the mass of one or both of the attracting masses larger, then the force will be larger. (Weight on Jupiter is larger than weight on Earth because the mass of Jupiter is so much larger than the mass of Earth.) The gravitational attraction between Earth and a satellite gets smaller as the satellite moves farther away from Earth. 3. Near Earth’s surface, the distance between the object that is falling and the center of Earth does not change very much. Also, the mass of Earth is constant. Thus, the force given by the law of universal gravitation depends only on the mass of the object that is falling. 4. Orbital motion has two components—horizontal and vertical. Horizontal motion propels the object forward, and vertical free fall pulls the object downward toward the center of gravity of the larger mass. 5. Answers may vary, but they should state something similar to the following: Newton’s second law shows that acceleration depends on both force and mass. A heavier object experiences a greater gravitational force than a lighter object (as you can see from the law of universal gravitation). But a heavier object is also harder to accelerate because it has more mass. The extra mass of the heavy object exactly compensates for the additional gravitational force. Since F = ma (or a = F/m), if F is increased at the same rate as m, then a remains the same. 6. The force of gravity is inversely proportional to the square of distance. Since the distance is made twice as close, the force of gravity will be four times as great (the square of 2), or 4 million N. Section 3 1. Newton’s third law states that anytime one object applies a force to a second object, the second object applies a force on the first object that is equal in size and opposite in direction. For example, when you sit in a chair, you push down on the chair (action force), and the chair pushes up on you (reaction force). 2. Momentum is mass _ velocity. 3. The law of conservation of momentum says that in any system or group of objects, the momentum will not change if a net outside force does not act on the system or group of objects. 4. b 5. Answers may vary. They can include any actionreaction force pairs, such as fingers and keyboard (on computer or piano, etc.), cheese and cheese cutter, person and floor, etc. 6. The forces exerted by Earth and a skier are an action-reaction force pair because Earth exerts an equal and opposite force on the skier as the skier had exerted on Earth. Since F = ma and F is equal for both, the smaller mass of the skier (when compared to the mass of Earth) will mean the skier’s acceleration away from Earth will be very much greater than Earth’s unnoticeable acceleration away from the skier. 7. p = mv = (1.0 kg)(12 m/s) = 12 kg•m/s eastward End of chapter Review Answers Using Key Terms 2. Inertia is the behavior of matter that defines mass in the laws of motion. It is important because inertia resists the effects of a net force. The more mass or inertia an object has, the smaller the acceleration for a given net force. 3. The wrestler will weigh less on the moon than he does on Earth, because the force exerted on him will be different at these locations. Since he has the same mass in both places, his weight will depend on the acceleration due to gravity at each location. The moon causes a much smaller acceleration due to gravity, so the wrestler will weigh less there. 4. As a skydiver jumps from a plane, gravity pulls her downward. Air resistance pushes upward against the downward motion. The skydiver accelerates downward until the force of air resistance equals the downward force of gravity. Then the skydiver stops accelerating and falls downward at a constant speed. This is called terminal velocity. 5. The ball on the far right will undergo a collision with the four stationary balls. Conservation of momentum requires that the ball on the far left will rise to a height equal to that at which the ball on the far right started. Understanding Key Ideas 6. c 7. d 8. a 9. b 10. b Math Skills 21. F = ma = 12.5 kg _ 3.5 m/s2 = 44 N 22. w = mg/6 = (2.26 kg _ 9.8 m/s2)/6 = 3.7 N 23. p = mv (65 kg)(3.0 m/s forward) = 195 kg•m/s forward (20.0 kg)(22 m/s west) = 440 kg•m/s west (16 kg)(0 m/s) = 0 kg•m/s