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Basic Moments Question

  1. Mar 25, 2007 #1
    1. The problem statement, all variables and given/known data
    Two plasterers Bill and Dave of mass 45kg and 65kg respectively stand on a unofrm plan ABCDEF of length 4m and mass 40kg. The plank is supported at C and E where C is 1m from A and E is 1m from F. Plasterer Bill stands at B which is 0.5m from A and plasterer Dave stands at D which is 1.5m from F.
    a) Draw a diagram showing the above information
    b) Find the reaction at each support, giving your answers in N to 1 decimal place.

    2. Relevant equations
    I drew this on paint, seems to be correct. Thats a) done.

    3. The attempt at a solution
    The question dosen't state anything about Equilbirum, but heres what I done:
    0.5 x 45g + S = 40g + 1.5 x 65g
    S = 40g + 97.5g - 22.5g
    S = 1127N

    Sub this into:
    S + R = 150g
    R = 150g - 1127
    R = 343N.

    However they are both completly wrong, I assume I am going wrong with thinking it is in equilbrium. Since it isn't, I'm not too sure on where to start.
    Any hints/help is very appreciated =)!

    Thank you.
  2. jcsd
  3. Mar 25, 2007 #2


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    I think it is correct to assume that the plank is in equilibrium, since it says it is supported at C and E.

    How have you set up your equations? The way to proceed in this sort of question is to take moments about a pivot, set the anticlockwise moments equal to the clockwise moments, and then obtain an equation. Now, to find S, say, where do you think you should take the pivot to be?
  4. Mar 25, 2007 #3

    Ahhh, I just found my mistake.
    I just put S, and S is 2m away from R.

    0.5 x 45g + 2S = 40g + 1.5 x 65g
    2S = 40g + 97.5g - 22.5g
    2S = 1127N
    S = 1127/2
    S = 563.5N

    R = 150g - 563.5
    R = 906.5N

    Those are the correct answers, I looked at it a few times and missed this out every time, THANK YOU for the help =)
  5. Mar 25, 2007 #4


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    Ahh, I didn't notice your m(R), so didn't quite get what you were doing! Sorry-- I guess you had already done everything I said in my previous post! Ahh well, you've got it now!
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