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# ch4 - CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION...

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CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION CONCEPTUAL QUESTIONS ______________________________________________________________________________________ 1. REASONING AND SOLUTION When the car comes to a sudden halt, the upper part of the body continues forward (as predicted by Newton's first law) if the force exerted by the lower back muscles is not great enough to give the upper body the same deceleration as the car. The lower portion of the body is held in place by the force of friction exerted by the car seat and the floor. When the car rapidly accelerates, the upper part of the body tries to remain at a constant velocity (again as predicted by Newton's first law). If the force provided by the lower back muscles is not great enough to give the upper body the same acceleration as the car, the upper body appears to be pressed backward against the seat as the car moves forward. ______________________________________________________________________________________ ______ 3. REASONING AND SOLUTION If the net external force acting on an object is zero, it is possible for the object to be traveling with a nonzero velocity. According to Newton’s second law, Σ F = m a , if the net external force Σ F is zero, the acceleration a is also zero . If the acceleration is zero, the velocity must be constant, both in magnitude and in direction. Thus, an object can move with a constant nonzero velocity when the net external force is zero. 6. REASONING AND SOLUTION Since the father and the daughter are standing on ice skates, there is virtually no friction between their bodies and the ground. We can assume, therefore, that the only horizontal force that acts on the daughter is due to the father, and similarly, the only horizontal force that acts on the father is due to the daughter. a. According to Newton's third law, when they push off against each other, the force exerted on the father by the daughter must be equal in magnitude and opposite in direction to the force exerted on the daughter by the father. In other words, both the father and the daughter experience pushing forces of equal magnitude. b. According to Newton's second law, F = m a . Therefore, a = F / m . The magnitude of the net force on the father is the same as the magnitude of the net force on the daughter, so we can conclude that, since the daughter has the smaller mass, she will acquire the larger acceleration. ______________________________________________________________________________________ ______ 10. REASONING AND SOLUTION The mass of an object is a quantitative measure of its inertia. The mass of an object is an intrinsic property of the object and is independent of the location of the object. The weight of an object is the gravitational

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force exerted on the object by the earth. The gravitational force depends on the distance between the object and the center of the earth. Therefore, when an object is moved from sea level to the top of a mountain, its weight will change, while the mass of the object remains constant.
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ch4 - CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION...

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