Lecture_19_web_2007

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Lecture 18 in a nutshell With friction ext W PE KE TE = ∆ + ∆ + ∆ Thermal energy made by friction Alternatively ext f W W PE KE - = ∆ + ∆ f k W F x = W f is negative because F k points in the opposite direction to the motion
Power

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Power work done Power time = ext F ext F x ext ext W F x = . 0 t f t 0 ext ext f F x F x Power t t t = = - . . J/s Watts Units = =
Power A person pushes a broken motor bike up a hill and in the process does 81160J of work. If it takes five minutes to get up the hill what is the average power of the person. W F x = . W Power t = 81160 5 60 Power = × 270.5Watts =

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Power An athlete who weighs 62kg can run up stairs, a total of 8 meters, in 7 seconds, assume their speed is constant. Q. what is the power of the sporty person W Power t = Mgh t = 62 9.8 8 7 × × = 694Watts =
Power A 2000kg car decelerates from 80to 70km/h in 6 seconds while in neutral without applying the breaks. Q1. Find the constant frictional force in the engine slowing the car down.

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