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Homework Help: Help please, power (piano pulled up by workers through pulley)

  1. Oct 28, 2008 #1
    A 3.50 kN piano is lifted by 3 workers at constant speed to an apartment 25 m above ground. Each worker is able to deliver 165 W of power and the pulley is 75% efficient (25% lost due to friction on pulley). Neglecting mass of pulley, what is the time required to lift the piano from the street to the apartment.
    I know that F=T-mg=ma=0
    T=3500N
    Work=3*165=495W=F/t
    But I'm not really sure how to sort this problem out and incorporate the 75% efficiency of the pulley and the height above ground. Please help me.
     
  2. jcsd
  3. Oct 28, 2008 #2

    Redbelly98

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    Not quite. You are equating work (units: Joules) with power (units: Watts) here, but they are not the same. And neither of those would equal force/time.

    At any rate, 495W is the power exerted by the 3 people. So how much of that power gets transmitted to the piano? (hint: it is not 100% of the power)
     
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