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Seriously? Moment of Force about a specified Axis

  1. Dec 30, 2007 #1
    [SOLVED] Seriously? Moment of Force about a specified Axis

    So I really thought I understood this. I have attached the problem and attempt at solution. It should be easy to follow.


    Problem
    [​IMG]
    Attempt at Solution
    This was my process:

    1. Write each F and each r as cartesian vectors
    2. Find each moment by cross product.
    3. Add the moments to get resultant moment M_R. Find unit vector along Oa axis=U_oa
    4. Use resultant Moment and Unit vector along Oa to find
    M_R dot U_oa=|M_oa|
    5. Write as Cartesian vector-->|M_oa|*U_oa
    [​IMG]
    The answer is supposed to be 26.1i-15.1j lb*ft. As you can see, my answer isn't close (WTF else is new!)

    What is wrong with my method?
    Thanks,
    Casey
     
  2. jcsd
  3. Dec 30, 2007 #2
    I can't believe I keep getting the same wrong answer!!!!
     
  4. Dec 30, 2007 #3
    I think for Moment 2 you suppose to only have (i) term
     
  5. Dec 30, 2007 #4
    Look at your second cross product again. Don't get discouraged, :-).

    Imagine attaching a flexible pipe from the origin to the a point on the line of action of F2 and pulling on it with a rope in the direction of f2, the pipe would begin to trace out a disk and the direction of the moment caused by F2 about the origin would be defined as being perpendicular to this disk and hence must only have an i component.
     
    Last edited: Dec 30, 2007
  6. Dec 30, 2007 #5
    Thanks guys! I was worried that my method was wrong. I wonder if I will EVER stop making tiny stupid errors. I just can't seem to stop...no matter how careful and aware I think I am being.

    Oh well...siggghhh.
    Thanks again!
     
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