A 3.50-kN piano is lifted by three workers at constant speed to anapartment 25 m

Question

A 3.50-kN piano is lifted by three workers at constant speed to anapartment 25 m above the street using a pulley system fastened tothe roof of the building. Each worker is able to deliver 165W ofpower, and the pulley system is 75% efficient (so that 25 % of themechanical energy is lost due to friction in the pulley).Neglecting the mass of the pulley, find the time required to liftthe piano from the street to the apartment.

I don't know how to put the 75% information into the problem tosolve it...

Explanation / Answer

power of one person P = 165 W Power of three workers P ' = 3 P                                           = 495 W power used to lift the piano P " = 75 % of P'                                                 = 0.75 * P '                                                 = 371.25 W work done to lift the piano from ground to 25 m above is W ' = weight of the piano * height                                                                                               = 3.5 k N * 25 m                                                                                               = 3500 N * 25 m                                                                                               = 87500 J we know power P " = W ' / time from this time   t = W ' / P "                         =235.69 s                        = 3.928 min

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