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Homework Help: Finding work for lifting a chain with winch on top of a building

  1. May 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A winch is positioned on top of a building, a distance 70 m above ground level. A chain of length 95 m and a mass per unit length of 1.2 kg/m hangs from the winch along the side of the building. Find the work done (in Joules) in reeling up 60 m of the chain.


    2. Relevant equations

    W=FxD

    F=μLg (according to a few examples)

    3. The attempt at a solution

    hmm i followed a few examples seeing if it can help me get to the right answer
    ok so i know that
    dW=F*dy F=μLg
    so μ=1.2, g=9.81, L=(70-y) ....got that from the equation but not too sure about L
    ....if i put that together to solve for W
    W=∫ μLg dy from 0 to 60 ....60 because its reeling it up by 60
    then i do all the integral magic and i get 28252.8
    but the answer is suppose to be 42200 >_<

    i used most of the number except for the length of the chain....not too sure what to do with it... i thought i can use to for my limits 95-60 and 95-70 but...i think thats just saying im only lifting its 25m to 35m =/


    hope i got the right idea....thanks
     
  2. jcsd
  3. May 16, 2010 #2

    lanedance

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    Homework Helper

    this one may be easier to balance work & potential energy (ps i didn't go through all the working)
     
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