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Unformatted text preview: hernandez (ejh742) – oldhomework 16 – Turner – (58120) 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 1260 N operating at a rate of 140 kW develops a maximum speed of 37 m / s on a level, horizontal road. Assuming that the resistive force (due to friction and air resistance) remains constant, what is the car’s maximum speed on an incline of 1 in 20; i.e. , if θ is the angle of the incline with the horizontal, sin θ = 1 / 20 ? Correct answer: 36 . 394 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 . 4 × 10 5 W 37 m / s = 3783 . 78 N . On the incline, the resisting force is F = f + mg sin θ = f + W 20 = P v h + W 20 . And, F v = P , so v = P F = P P v h + W 20 = 1 . 4 × 10 5 W 1 . 4 × 10 5 W 37 m / s + 1260 N 20 = 36 . 394 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 11 m / s? Correct answer: 43007 . 6 W. Explanation: On the reduced incline, we again have, P = F v = ( f + m g sin θ ) v The only change from the previous problem is sin θ = 1 10 , so P = parenleftbigg f + W 10 parenrightbigg v = parenleftbigg p v h + W 10 parenrightbigg v = parenleftbigg 1 . 4 × 10 5 W 37 m / s + 1260 N 10 parenrightbigg (11 m / s) = 43007 . 6 W . 003 10.0 points A certain waterfall is 137 . 6 m high and has water flow rate of 28 . 8 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 . 88362 × 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 Δ U = ( ρr ) g h = (1000 kg / m 3 ) (28 . 8 m 3 / s) × (9 . 8 m / s 2 ) (137 . 6 m) = 3 . 88362 × 10 7 J / s . hernandez (ejh742) – oldhomework 16 – Turner – (58120) 2 When converted into electric energy, the power is P = Δ U = 3 . 88362 × 10 7 W . 004 10.0 points A 131 kg physics professor has fallen into the Grand Canyon. Luckily, he managed to grab a branch and is now hanging 76 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 06/14/2011 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas.
 Spring '08
 Turner
 Physics, Power, Work

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