How long will it take a 2750-W motor to lift a 385-kg piano to a sixth-story window 16.0 m above?
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This is Giancoli Answers with Mr. Dychko. Power is work divided by time and if we assume that piano is being lifted at constant speed, the amount of work done will be the force due to gravity times the distance because the motor's gonna have to pull up with the force equal to the force downwards of gravity. So here's the piano and here's the motor applying some tension force. And as it's going at constant speed, the tension force will be equal to the gravity down and so tension force gonna be equal to m g. So that times the distance over which the force acts, which is the height that it's being lifted, divided by P is gonna work out to be t. So we can multiply both sides by t over P and solve for t. So t is w over P; the work done is the force times the distance and the force is m g times the height over P so that's 385 kilograms times 9.8 newtons per kilogram times 16 meters up divided by 2750 watts which is 22.0 seconds when you round that to three significant figures.
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