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Homework Help: Forces and Moments acting on an entire system.

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data
    The ceiling-mounted bicycle rack (shown in "Capture" image in attachments) with dimensions p = 50 mm, q = 1000 mm and r = 150 mm for a bicycle that weighs F = 200 N.

    The rack is to be made from two parts, ABC and BD, cut from a steel tube and then welded together at B.

    Determine the forces and moments acting on the whole rack, ABCD.

    2. Relevant equations

    The only equations that I can see would be relevant here are the force and moment equations:
    M = (F*d)
    Ʃ M = 0
    Ʃ F = o

    3. The attempt at a solution

    I have found M(of B) and M(of C) by doing the following:

    M(of B) = 200*0.15 = 30Nm Clockwise

    Therefore, 30 = 0.05*F(at A), so F(at A) = 600N

    So... M(of C) = 1.05*600 = 630Nm Clockwise.

    So I have found M at points B and C, and the force at A, however I am not confident that is what's required of the question. What does it mean by "the forces and moments acting on the whole rack ABCD"??

    Attached Files:

  2. jcsd
  3. Nov 14, 2012 #2


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    There seems to be something missing in the problem statement. I gather from your attempted solution that there is a a horizontal force FA at A.
    If you are taking moments about B then you must consider a contribution from the force at C. It does not necessarily act vertically. Better might be to take moments about C.
  4. Nov 14, 2012 #3
    There is no acting force on A. I just calculated that because if there is a moment at B, then having the 200N force at D would be the equivalent of having a Force of 600N at A. Am I wrong?
  5. Nov 14, 2012 #4


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    Yes, that'd be wrong. If there's no force acting below B then the piece of the rack below B is irrelevant. Just take moments about C.
    The question is fairly clear: what are all the forces and moments acting on the rack, assuming it stays in balance?

    All that said, the question looks strange to me. It would be more reasonable if there were a horizontal force at A.
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