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Unformatted text preview: alexander (jra2623) oldhomework 14 Turner (92510) 1 This printout should have 11 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 2) 10.0 points A car of weight 2060 N operating at a rate of 172 kW develops a maximum speed of 25 m / s on a level, horizontal road. Assuming that the resistive force (due to friction and air resistance) remains constant, what is the cars maximum speed on an incline of 1 in 20; i.e. , if is the angle of the incline with the horizontal, sin = 1 / 20 ? Correct answer: 24 . 6312 m / s. Explanation: If f is the resisting force on a horizontal road, then the power P is P = f v horizontal . so that f = P v h = 1 . 72 10 5 W 25 m / s = 6880 N . On the incline, the resisting force is F = f + mg sin = f + W 20 = P v h + W 20 . And, F v = P , so v = P F = P P v h + W 20 = 1 . 72 10 5 W 1 . 72 10 5 W 25 m / s + 2060 N 20 = 24 . 6312 m / s . 002 (part 2 of 2) 10.0 points What is its power output on a 1in10 incline if the car is traveling at 14 m / s? Correct answer: 99204 W. Explanation: On the reduced incline, we again have, P = F v = ( f + m g sin ) v The only change from the previous problem is sin = 1 10 , so P = parenleftbigg f + W 10 parenrightbigg v = parenleftbigg p v h + W 10 parenrightbigg v = parenleftbigg 1 . 72 10 5 W 25 m / s + 2060 N 10 parenrightbigg (14 m / s) = 99204 W . 003 10.0 points A certain waterfall is 118 . 7 m high and has water flow rate of 30 . 2 m 3 / s. The acceleration of gravity is 9 . 8 m / s 2 . Find the maximum electric power that can be generated by these falls assuming 100% conversion of mechanical energy to electric energy. (Take the density of water to be 1 . 00 10 3 kg / m 3 ) Correct answer: 3 . 51305 10 7 W. Explanation: Basic Concepts: Potential Energy and Power Solution: When water falls from a high place, its potential energy decreases; for the waterfall, the variation of potential energy per second is U = ( r ) g h = (1000 kg / m 3 ) (30 . 2 m 3 / s) (9 . 8 m / s 2 ) (118 . 7 m) = 3 . 51305 10 7 J / s . alexander (jra2623) oldhomework 14 Turner (92510) 2 When converted into electric energy, the power is P = U = 3 . 51305 10 7 W . 004 10.0 points A 103 kg physics professor has fallen into the Grand Canyon. Luckily, he managed to grab a branch and is now hanging 98 m below the rim. A student (majoring in lin guistics and physics) decides to perform a rescue/experiment using a nearby horse. Af ter lowering a rope to her fallen hero and attaching the other end to the horse, the stu dent measures how long it takes for the horse to pull the fallen physicist to the rim of the...
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This note was uploaded on 10/20/2011 for the course PHY 302K taught by Professor Kaplunovsky during the Summer '08 term at University of Texas at Austin.
 Summer '08
 Kaplunovsky
 Power, Work

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