So I have this problem and I can't move on until i got this assumption cleared up. Looking at this truss(adsbygoogle = window.adsbygoogle || []).push({});

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 byinspectionthat C_x and B_x are in the same direction (in the negative x direction).But, for the sake ofconsistencyI 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?

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# Homework Help: PLEASE help Simple question on FBD assumption

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