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Homework Help: Find the Moment (Torque) of the resultant in 3-D (Statics)

  1. Mar 2, 2010 #1
    Hey guys... This problem is driving me bonkers. I really am not asking you to calculate anything but I only ask how to figure out how to get r to use τ = r x F . My professor did not even teach us anything about resultants and 3-d moments :grumpy:

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


    2. Relevant equations

    MO= τ = r X F

    3. The attempt at a solution

    Well, I found that the resultant F = 288.55 i + 985.97 j + 437 k .
    The resultant is CORRECT. I can not figure out how to get the distance of this vector in respect to x, y , z for r so I can use the cross product to figure out the moment.

    ...Or perhaps I have approached the problem all wrong? Is there a different way to solve the problem? ( please do not use any numbers just to make it easier on you)

    The book makes no mention of how to solve this problem and it is so so frustrating because even the professor has not done any problems pertaining to this question.
    Last edited: Mar 2, 2010
  2. jcsd
  3. Mar 2, 2010 #2


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    Staff: Mentor

    If you are to find the torque about the origin, then the r vector would just be from the origin to the point that the force is applied (where the ropes are attached). It's a little hard to read the picture, but that would be my impression.
  4. Mar 2, 2010 #3
    So the r vector will just be 0i + 7j + 0k? I will test it out right now if it works... the answer is in the back of the book.. So I will get back with the results in a sec
  5. Mar 2, 2010 #4
    well, the answer to the problem is:

    3080 i - 2070 k

    however my answer is:

    - 3059 i - 2019 k

    maybe my resultant vector is slightly off? And cross product makes i negative =-\
  6. Mar 3, 2010 #5


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    Staff: Mentor

    Can you show us your cross product math?
  7. Mar 3, 2010 #6
    XX Take your moment about point O caused by one of the forces and sum it with the moment about point O caused by the other forces
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