- #1
Saladsamurai
- 3,009
- 7
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?
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?
