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Homework Help: Piano and krane

  1. Oct 8, 2006 #1
    A piano whose mass is 132 kg is being lifted at a steady speed from ground level straight up to an apartment 7.3m above the ground. The crane that is doing the lifting produces a steady power of 470W. How much time does it take to lift the piano?

    Sorry for so many topics, I do appreciate the help though!!

    anyway, I know that the sum of KE and PE on the ground should be equal to KE and PE at the top ..

    KEground = .5 * 132 * v^2
    PEground = 0

    KEtop = .5 * 132 * v^2
    PEtop = 9452.916

    My question is how do I find the time it takes to lift the piano if I don't know what the velocities are? thanks
  2. jcsd
  3. Oct 8, 2006 #2
    Perhaps it would be helpful to think of power as the rate at which work is done. Also, think about what units work is measured in.

    You mentioned "velocities"... constant velocity; v = v.

    Your error is thinking that the sum of the KE and PE at the bottom is the same as the sum of the KE and PE at the top. There's one thing missing from that equation...
  4. Oct 8, 2006 #3
    I thought that energy being conserved meant that the energy from the point of origin was equal to the energy at its destination...since this isn't true I'm unsure as to what I'm missing :confused: ?
  5. Oct 8, 2006 #4
    The machine is doing work on the system... i.e. it's adding energy.
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