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# hw10 - lavallo(jhl936 Homework 10 orin(58140 This print-out...

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lavallo (jhl936) – Homework 10 – florin – (58140) 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A weight lifter lifts a mass m at constant speed to a height h in time t . What is the average power output of the weight lifter? 1. ¯ P = m g h t correct 2. ¯ P = m h 3. ¯ P = m g h t 4. ¯ P = m g h 5. ¯ P = g Explanation: The amount of work to lift a mass m to a height h is m g h . The power is the amount of work done divided by the time t . Alternately, power has units of energy over time. The only answer with the correct units is ¯ P = m g h t . 002 10.0 points Units of power include which of the following? I) Watt II) Joule per second III) Kilowatt-hour 1. I only 2. III only 3. II and III only 4. I, II and III 5. I and II only correct Explanation: [Power] = [Energy] [time] Watt = Joule second is the unit of power. Kilowatt-hour is a unit of energy. Thus, only I and II are correct. 003 10.0 points A car weighing 6500 N moves along a level highway with a speed of 81 km / h. The power of the engine at this speed is 78 kW. The car encounters a hill inclined at an angle of 8 . 1 with respect to the horizontal. If there is no change in the power of the engine, and no change in the resistive forces acting on the car, what is the new speed of the car on the hill? Correct answer: 64 . 0726 km / h. Explanation: Since the velocity is constant, the car must be in equilibrium. Assume F is the pulling force provided by the engine and f is the fric- tional force. On the level highway F 1 = f F 1 P v On the hill: F 2 = f + mg sin( θ ) v 2 P F 2 = F 1 v F 2 = F 1 v F 1 + m g sin

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