Mastering Vector Direction: How to Solve for Unknown Forces in Your Equations

[Ry122]

How do you know what direction to make vectors A, B and C face when formulating your equation? I guess its unknowable really isn't it?

therefore for the sum of forces in x direction you'd just say C + P + B_x = 0

How about if it is knowable? E.g. if B didn't exist and therefore you'd know C definitely points in the opposite direction to P_x.
If you then modeled your equation the same as above, and just ignoring signs like this, would you still get the correct answer? P + C = 0

 

[cpsinkule]

I'm having some trouble reading your diagram. What direction are A and B pointing

 

[Ry122]

I'm having some trouble reading your diagram. What direction are A and B pointing

My point is that it's unknowable

 

[SteamKing]

My point is that it's unknowable

Yes, it is, given how vaguely worded the OP was.

You mention "your equation", but I haven't the foggiest idea what you are talking about.

 

[Ry122]

Okay, I'll reword my question. I guess you just need to make an assumption to begin with about which way each vector faces, right? Let's say A and B both point towards the joint, but this turns out to be wrong. How do you know you were wrong?

 

[cpsinkule]

if your figure is a free body diagram then you need no assumptions. you need to project your known forces onto some axes and use the requirements for static equilibrium ( i assume that is what this problem is) ΣF=0 for each basis direction

 

[donpacino]

just take a guess. if your guess is "wrong" you will get a negative number