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Homework Help: Finding 3D moment

  1. Sep 13, 2015 #1
    1. The problem statement, all variables and given/known data

    A (0,0,0)

    |F|= 1919N

    2. Relevant equations

    3. The attempt at a solution

    B= {5i + 6j + 1k}
    C= {3i + 4k}

    rCB ={2i + 6j - 3k}

    |rCB| = sqrt(22 + 62 + 32) = 7

    Unit vector = rCB / |rCB| = {2/7i + 6/7j - 3/7k}

    F = |F| * Unit vector = 1919 {2/7i + 6/7j - 3/7k} = 548.29i + 1644.86j - 822.43k

    Now firstly, I'm not sure if I even did the above part correctly, but to find the moment I'm a bit lost and my don't seem to have much in my notes about this...

    I think the next step is using cross product, but I'm a bit lost as to which arm to take to find moment about A...

    If I use rBA I get:

    MA = rBA x F = 6579.44i - 4660.44j - 4934.56k

    WELP! Need help!!

    Cheers LB
  2. jcsd
  3. Sep 14, 2015 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    This is not the r-vector for use in calculating the moment about point A. What is the r-vector you need instead?

    (Hint: What is the Force pulling on?)
    You can always find discussions and examples of how to calculate the moment on the web.
    See above. Remember, the problem asks for the magnitude of the moment, so don't forget that!
  4. Sep 14, 2015 #3
    So I need to use rBC instead?
  5. Sep 14, 2015 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    No, you need to use the r-vector which connects point A to the end of the rope pulling on the end of bar AB.

    Remember, calculating a Moment requires a Force and some kind of Distance between the force and the axis of rotation caused by the moment.
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