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Help!Find the total moment.

  1. Oct 28, 2013 #1
    I heard that the moment of F about point a is (r-r(a))xF
    but i can't understand what r and r(a) are.And when there are 2 forces act though different point,how can I find the total moment.

    Four points:O(0,0,0) P(9,-3,5) Q(6,4,7) R(3,2,3.5).
    OPQ is a triangular plane and R is a point on this plane.
    Two forces, F(1)=i-2j+k and F(2)=3i+3j+2k act through the points P and Q respectively.
    If the triangle OPQ is fixed at the point R, what is the total moment of the two forces about it??

    I do it like that:(OP-OR)xF(1)+(OQ-OR)xF(2)
    Is that right?
  2. jcsd
  3. Oct 28, 2013 #2


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    hi go2255! welcome to pf! :smile:
    r is the point of application of the force (or any point on the line of application)

    r(a) is the point about which you're taking the moments

    in fact, you've got it correct in your example :wink:
    you add the two moments (as vectors)
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