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Homework Help: Work problem

  1. Feb 29, 2008 #1
    [SOLVED] Work problem

    A vertical rope is used to lower a mass 44.9 kg at a constant acceleration of magnitude 4.57 m/s2. Find the work done by the tension in the cord if the mass moves down distance 2.66 m.

    W = F (change in X)

    First, I tried to get the tension by:

    -T + mg = ma
    - T = ma - mg
    T = -ma + mg
    T = - (4.57 m/s^2)(44.9 kg) + (44.9 kg) (9.81 m/s^2)
    T = 235.276 N

    W = T (change of x)
    W = 235.276 N (2.66 m) = 625.83416

    However this seems to be wrong so my second attempt

    T -mg = ma
    T = ma + mg
    T = 645.662 N

    W = T (change of x)
    W = 645.662 N (2.66 m) = 1717.46092

    It seems I'm doing something wrong or overlooking a vital piece of information, I was wondering if someone could point me in the right direction.
  2. jcsd
  3. Feb 29, 2008 #2
    The first attempt looks good to me, except that the work done would be negative.
  4. Feb 29, 2008 #3


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    Science Advisor
    Homework Helper

    I can't find anything wrong with your first attempt. It looks correct. The second attempt IS wrong. It would be right if mass were accelerating 4.57m/sec^2 in an upwards direction. But it doesn't look like that's what the question is asking. Have you tried just changing the number of significant figures in the answer?
  5. Feb 29, 2008 #4
    yeah it seems i was using the incorrect direction of the tension since it is opposite of the weight then it should be negative. Thanks.
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