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Homework Help: 3D Forces and Moments Problem.

  1. Apr 21, 2013 #1
    Forces F1 =j + 3k, F2 = j + k and F3 = 2i + j act at the point (0; 1;2), (1;0;3) and (1;1;0) respectively.
    (a) Find the total force and the total moment about the origin.
    (b) Find an equivalent system of forces consisting of a single force acting at the (1;0;0) together with a couple.

    Ok so for (a), I think I have the correct answer. For the total force I simply added up the sum of forces and got a figure of -2i + j + 4k.
    Then for the total moments I got the sum of each of the 3 individual moments. I calculated these by computing the determinant of a 3x3 matrix, with the top row being i j k , middle row being 'r' and the bottom row being the force. I got the total moment to be -4i + j + 2k.

    However, I'm stuck on (b). I don't really know where to start. If someone could write out how to do it I'd appreciate it so much because my Applied Mathematics exam is tomorrow !
  2. jcsd
  3. Apr 21, 2013 #2


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    Homework Helper

    You'd do it in the same way as part a)

    Except you'd need to get the new 'r' vectors from the points to <1,0,0> instead of the origin.

    Use r = rA-rB
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