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Homework Help: Torque Truss Question

  1. Jan 31, 2006 #1
    Okay, I have uploaded the question to imageshack because it would be to difficult to understand the question without the diagram. Basically, I've done a lot of work on it and know I'm on the right track. If someone could point out where I have made a mechanical error it would be greatly appreciated. The question I am on is number 3.22, the one in the top right corner of the page.

    Go here for picture of question.

    So first off I made a free body diagram. Two Fn's going up, two 150's going down and a 500 going down. I solved for Fn and got 400N.

    Then I solved the triangle to find that the angle between the cord and the board is 54 degrees. Finally I did torque calculations as follows:

    T1 = 400(Fn) * 3 * sin 36
    T1 = -705N

    T2 = T * 2.5 * sin 54
    T2 = 2.02T

    2.02T = 705
    T = 349N?

    The answer should be 280 Newtons. Where have I gone wrong?
  2. jcsd
  3. Jan 31, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What about the torque due to the weight of the rafters?
  4. Jan 31, 2006 #3
    Damnit. I knew I forgot something.

    Haha thanks bud!
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