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Homework Help: Centroid of trapezoid

  1. Aug 27, 2007 #1
    Virtual work (fireplace thong)

    1. The problem statement, all variables and given/known data

    I have been given a fireplace thong and need to determine the force exerted on a log.

    2. Relevant equations
    It is in the virtual work chapter of my textbook but I cannot see how the equations they have given me relate to the question.

    3. The attempt at a solution
    see above
    Last edited: Aug 27, 2007
  2. jcsd
  3. Aug 27, 2007 #2


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    Think of it as a series of levers ( like individual see-saw or teeter-totter )

    ps I think you mean 'tong', fireplace thong sounds very painful.
  4. Aug 29, 2007 #3
    Still cant see how the equation U=intg[F costeta ds] or U=int[M dteta] apply to situation.
    help appreciated please.
  5. Aug 29, 2007 #4


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    One has to work with moments (applied force and moment arm). F is applied about A and induces a moment (force) at C. C then applies a moment about D at N.

    There are angles involved, since F is applied at some angle with respect to the moment arm (FA). And the force at C (from the moment with respect to AC) acts at an angle with respect to the moment arm CD.

    Statically the moments induced by FB must be equal and opposite FA.
  6. Aug 29, 2007 #5
    Didnt really understand your explanation but i was just playing around with the numbers and seem to have got the anwser

    24/17 x17/17 x 17/6=N=4F

    thats the right anwser but stiil dont understand it.
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