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Homework Help: Shear Force and Bending Moment Diagram

  1. Nov 29, 2009 #1
    1. The problem statement, all variables and given/known data

    I have to find the Shear Force and Bending Moment diagrams for this problem. The lengths in the picture are 1.2m each and the peak force is 4kN/m and I calculated the support reactions to be:

    Ay=4,8kN (up)
    Ma= 5.76kN.m (anticlockwise)

    2. Relevant equations

    These are what I'm looking for but failed to do...

    3. The attempt at a solution

    I first section the beam 0<x<4


    +4.8 -.5x*3.33x-Vx=0

    I got the 3.33x by using similar triangles; if triangle 1 is (1,2;4) then sectioned triangle should be (x;y) so therefore 4/1.2=y/x x=3.33x

    this gave me Vx=-1.67x^3+4.8 which is wrong.

    Then EMo=0

    +Mx +.5x*3.33x*(x/3) -4.8x +5.76=0

    which gets me Mx=-0.56x^3 +4.8x +5.76

    The correct answers for this question are:

    Vx=4.8 -(x^3/6)
    Mx=4.8x -(x^3/18) -5.76

    I have a feeling I'm not using the triangle force correctly. Please advise.

    Attached Files:

  2. jcsd
  3. Nov 29, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    jasonbot, welcome to PF!

    Is it 12 m or 1.2 m?
    yes, looks good
    0<x<12 m
    if you correct the 1.2 m to 12m , then Vx = -.167x^2 +4.8
    something is afoul between 1.2 m and 12 m, the decimal point is messed.., but you are still not quite right, don't let the plus and minus signs wear you down.......
    typo error
    taking clockwise moments as plus, I don't know about that decimal point, however

    Looks like you did well, but check out your plus and minus signs, they'll bite you every time....
    Last edited: Nov 29, 2009
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