Statics: Find resultant force and couple moment

  1. [​IMG]

    Calculate the resultant force’s magnitude and the couple moment.

    Let a = 3.70 m
    w1=5.35kN/m
    w2=4.00kN/m

    The resultant force of the triangle is 1/2(3.70)(5.35)=9.898 kN

    The resultant force of the rectangle is (4)(3.70) = 14.8kN

    So FR (The force's magnitude) is 24.69kN

    I found the location of the triangle's load to be 1/3(3.7) = 1.233m

    I found the location of the rectangles load to be 1/2(3.70)+3.7 = 5.550m

    So I calculated the couple moment MRa (Taking counter clockwise to be POSITIVE) I get
    (9.898)(1.233)+(14.8)(5.55) = 94.3kN*m

    but when I put these answers down for MasteringEngineering, they are incorrect. Where did I go wrong?
     
  2. jcsd
  3. haruspex

    haruspex 12,513
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    W1 and W2 are acting in opposite directions.
    Even if you correct that your FR calculation will still be wrong. You don't want the total vertical force. You only want the load at A. To get that, take moments about B.
    When you take moments about A for the second part, W1 has a negative moment. (I assume it's only asking for the moment about A due to W1 and W2. If the joint at B is holding the beam then there will also be a moment from that, and the total moment will be zero.)
     
  4. haruspex

    haruspex 12,513
    Science Advisor
    Homework Helper
    Gold Member
    2014 Award

    W1 and W2 are acting in opposite directions.
    Even if you correct that your FR calculation will still be wrong. You don't want the total vertical force. You only want the load at A. To get that, take moments about B.
    When you take moments about A for the second part, W1 has a negative moment. (I assume it's only asking for the moment about A due to W1 and W2. If the joint at B is holding the beam then there will also be a moment from that, and the total moment will be zero.)
     
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