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Homework Help: Statics equilibrium question

  1. Aug 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Question: The plate shown, picture is attached, is subjected to force F and 3 reaction forces R1 R2 R3. Find R1

    2. Relevant equations

    equations of equilibrium:
    sum of Fx = 0
    sum of Fy = 0
    sum of moment about point O = 0

    3. The attempt at a solution

    sum of Fx = 0: (going right is positive)

    -R1 - R2 + 3/5 R3 = 0

    sum of Fy = 0: (going up is positive)

    -56kN + 4/5 R3 = 0
    so, R3 = 70kN

    sum of moment about point O = 0 (going anticlockwise is positive)

    R1 x (0.11m) - R2 x (0.11m) = 0
    so, R1 = R2

    therefore, R1 = R2 = 21kN

    I am unsure of whether I have done this question correctly, in particular the moment about O.
    Also, how do you figure out which direction the moment is in (anticlockwise/clockwise). As in, I took R1 x 0.11m as positive due to anticlockwise and
    R2 x 0.11m as negative due to clockwise moment.
    is this correct?

    Thanks for any help you can offer!

    Attached Files:

  2. jcsd
  3. Aug 5, 2012 #2


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    Science Advisor
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    Gold Member

    R3=70Kn appears correct.

    As for the moment...

    R1*110 + (-R2*110) = 0


    R1 = R2

    Both same sign

    Horizontally, if left is positive...

    -R3*3/5 + R1 + R2 = 0

    -R3*3/5 + R1 + R1 = 0

    -R3*3/5 + 2R1 = 0

    R1 = R3*3/(5*2)

    = 21kN

    which is a +ve number so to the left.

    R1 = R2

    so R2 also to the left.
  4. Aug 5, 2012 #3
    Thank you!!
  5. Aug 6, 2012 #4
    To check whether you are correct, just take moments about any other point. The sum should be zero.
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