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Homework Help: Pulley Efficiency Problem (How much rope must be pulled)?

  1. Jan 12, 2012 #1
    1. The problem statement, all variables and given/known data
    A pulley system has an efficiency of 71.5 percent. How much of the rope must be pulled in if a force of 641 N is needed to lift a 160 kg desk 2.51 meters?

    2. Relevant equations

    IMA = De/Dr
    MA = Fr/Fe
    Efficiency (%) = Wo/Wi
    Efficiency (%) = MA/IMA

    I have absolutely no idea how to solve this; this is simply a practice problem that I made up and the professor said a similar problem will appear on the test. Could someone show me how to do this?? I would appreciate it greatly.

    3. The attempt at a solution
  2. jcsd
  3. Jan 13, 2012 #2


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    How much work is needed to lift the desk?

    Only 71.5% of the work done to the rope goes into lifting the desk.

    How much work needs to be does to the rope?
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