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# ch6 - CHAPTER 6 WORK AND ENERGY CONCEPTUAL QUESTIONS 2...

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CHAPTER 6 WORK AND ENERGY CONCEPTUAL QUESTIONS ______________________________________________________________________________________ ______ 2. REASONING AND SOLUTION The force P acts along the displacement; therefore, it does positive work. Both the normal force F N and the weight m g are perpendicular to the displacement; therefore, they do zero work. The kinetic frictional force f k acts opposite to the direction of the displacement; therefore, it does negative work. ______________________________________________________________________________________ ______ 4. REASONING AND SOLUTION The sailboat moves at constant velocity, and, therefore, has zero acceleration. From Newton's second law, we know that the net external force on the sailboat must be zero. a. There is no work done on the sailboat by a zero net external force. b. Work is done by the individual forces that act on the boat; namely the wind that propels the boat forward and the water that resists the motion of the boat. Since the wind propels the boat forward, it does positive work on the boat. Since the force of the water is a resistive force, it acts opposite to the displacement of the boat, and, therefore, it does negative work. Since the total work done on the boat is zero, each force must do an equal amount of work with one quantity being positive, and the other being negative. Note: The answer to part (a) could have been deduced from the work-energy theorem as well. Since the velocity of the boat is constant, the kinetic energy of the boat does not change and the total work done on the boat is zero. ______________________________________________________________________________________ ______ 6. REASONING AND SOLUTION The kinetic energy of an object of mass m moving with speed v is given by 2 1 2 KE mv = . The kinetic energy depends on both the mass and the speed of the object. The mass of an automobile is significantly greater than the mass of a motorcycle. Therefore, even if an automobile is moving slowly, it is possible that the product 2 1 2 mv is greater for the car than it is for the faster-moving motorcycle. ______________________________________________________________________________________ ______ 7. REASONING AND SOLUTION A net external force acts on a particle. This net force is not zero. From Newton's second law, we can conclude that the net external force causes the particle to accelerate. Since the particle experiences an acceleration, its velocity must change. The change in velocity, however, may occur as a change

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in magnitude only, a change in direction only, or a change in both magnitude and direction. a. This is sufficient information to conclude that the velocity of the particle changes; however, there is not sufficient information to determine exactly how the velocity changes. b. There is not sufficient information to determine if the kinetic energy of the particle changes. In terms of the work-energy theorem, the kinetic energy will change if the net external force does work on the particle. But without knowing the direction of the net external force with respect to the particle's displacement, we cannot know if work is done.
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