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SM_PDF_chapter4 - The Laws of Motion CHAPTER OUTLINE 4.1...

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83 The Laws of Motion CHAPTER OUTLINE 4.1 The Concept of Force 4.2 Newton’s First Law 4.3 Mass 4.4 Newton’s Second Law The Particle Under a Net Force 4.5 The Gravitational Force and Weight 4.6 Newton’s Third Law 4.7 Applications of Newton’s Laws 4.8 Context Connection Forces on Automobiles ANSWERS TO QUESTIONS Q4.1 (a) The force due to gravity of the earth pulling down on the ball—the reaction force is the force due to 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. Q4.2 Mistake one: The car might be momentarily at rest, in the process of (suddenly) reversing forward into backward motion. In this case, the forces on it add to a (large) backward resultant. Mistake two: There are no cars in interstellar space. If the car is remaining at rest, there are some large forces on it, including its weight and some force or forces of support. Mistake three: The statement reverses cause and effect, like a politician who thinks that his getting elected was the reason for people to vote for him. Q4.3 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 and the 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 falls into her lap. (Both performers won Academy Awards.) Q4.4 No. The reaction to your weight is an upward force you exert on the Earth. Q4.5 First ask, “Was the bus moving forward or backing up?” If it was moving forward, the passenger 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 fly far. Fine her for malicious litigiousness. Q4.6 It would be smart for the explorer to gently push the rock back into the storage compartment. Newton’s 3rd law states that the rock will apply the same size force on her that she applies on it. The harder she pushes on the rock, the larger her resulting acceleration. Q4.7 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.
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84 The Laws of Motion Q4.8 While a football is in flight, the force of gravity and air resistance act on it. When a football is in the
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