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Homework Help: PLEASE help Simple question on FBD assumption

  1. Jan 11, 2008 #1
    So I have this problem and I can't move on until i got this assumption cleared up. Looking at this truss

    If we look at the entire truss' FBD we have reactions in the x,y directions at
    B and a reaction in the x direction at C.

    Now I know by inspection that C_x and B_x are in the same direction (in the negative x direction). But, for the sake of consistency I will assume that all forces are in the POSITIVE direction. After solving my equations of equilibrium, I know that a negative sign will mean that the assumed direction was incorrect.

    I solved for C_x directly by taking the moments about B.
    [tex]\sum M_B=0\Rightarrow 1(10)+2(8)+2(C_x)=0\Rightarrow C_x=-13kN[/tex]

    The neg sign shows that C_x is indeed leftwards. Now to find B_x I used Sum of Forces.
    this time I used the REVISED sense of C_x and the ASSUMED sense of B_x:

    This gives me [tex]B_x-13kN=0\Rightarrow B_x=13kN[/tex] which seems to be a conflict. Why would B_x be positive. Clearly it should have the same sign as C_x.

    Where have I gone completely awry?
  2. jcsd
  3. Jan 11, 2008 #2


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    Staff Emeritus
    Science Advisor

    The x-reactions at B and C are in opposite directions. B is under tension, while C is in compression. The two load P1 and P2 pivot about B, pushing on C.

    In contrast, load P1 and P2 pivot about C pulling on B.

    One has to calculate the moments about B and C, due to P1, P2 and the other reaction.

    There is a vertical reaction at B, which is just the sum of P1 and P2, since the roller at C provides no reaction.
  4. Jan 11, 2008 #3
    Yeah Nuc!! Thanks! I have always trusted the math over my intuition and it is a problem such as this that confirms my trust in math over myself! It seems like I have to make every mistake that exists at least once before I can predict future ones of the like.

    Hopefully that will change, otherwise I will make a lethal engineer!:uhh:
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