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Cut section method in truss

  1. Mar 23, 2017 #1
    1. The problem statement, all variables and given/known data
    For moment about D , how could the FGF sin26.6 generate moment about D ?

    2. Relevant equations


    3. The attempt at a solution
    FGF sin26.6 is located above point D , right ? How could FGF sin26.6 generate moment about D ?
     

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  2. jcsd
  3. Mar 24, 2017 #2

    haruspex

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    If calculating moments by first breaking the force into components then you have to select a point of application of the force. Anywhere in the line of action will do. In this case you can use G, F or O.
    If you use G there will be a clockwise moment about D from the horizontal component and an anticlockwise moment from the vertical component.
    If you select F, only the horizontal component will have a moment about D, but it will be less than that of the horizontal component at G.
    If you select O, as the author has done, only the vertical component has moment about D.
     
  4. Mar 24, 2017 #3
    S
    Ok , i can understand what you have explained . But for this type of question ,
    how do we choose the point of application of force ? It's not stated in the question . Choosing different point of application of forces will yield different answer
     
  5. Mar 24, 2017 #4

    haruspex

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    No, as long as the points are along the line of action they will all yield the same answer.
     
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