Chapter4

Chapter4 - 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. ____________________________________________________________________________________________ 2. REASONING AND SOLUTION When the birdfeeder is hanging freely and no one is pulling on the dangling (lower) cord, there is a tension in the cord between the birdfeeder and the tree limb (the upper cord), because the upper cord supports the weight of the birdfeeder. When the lower cord is pulled down with a slow continuous pull, the tension in both cords increases slowly. Since the upper cord has a larger tension to begin with, it always has the greater tension as the lower cord is pulled. Thus, the upper cord snaps first. On the other hand, when the child gives the lower cord a sudden, downward pull, the tension in the lower cord increases suddenly. However, the tension in the upper cord does not increase as suddenly. The reason is that the birdfeeder has a large mass, so it accelerates very slowly. Thus, the upper cord is stretched slowly and, consequently, the tension in the upper cord rises slowly. Since the tension rises much faster in the lower cord, it is the first to snap. ____________________________________________________________________________________________ 3. SSM 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. ____________________________________________________________________________________________ 4. REASONING AND SOLUTION According to Newton's second law, a net force is required to give an object a non-zero acceleration. a. If an object is moving with a constant acceleration of 9.80 m/s 2 , we can conclude that there is a net force on the object.
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156 FORCES AND NEWTON'S LAWS OF MOTION b. If an object moves with a constant velocity of 9.80 m/s, its acceleration is zero; therefore, we can conclude that the net force acting on the object is zero. ____________________________________________________________________________________________ 5. REASONING AND SOLUTION An object will not necessarily accelerate when two or more forces are applied to the object simultaneously. The applied forces may cancel so the net force is zero; in such a case, the object will not accelerate. The resultant of all the forces that act on the object must be nonzero in order for the object to accelerate.
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Chapter4 - CHAPTER 4 FORCES AND NEWTON'S LAWS OF MOTION...

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