Truss problem (method of joints). Newbie needs help.?

[Marc]

As the title says, I'm a newbie with this topic and I'm kind of confused with the method joints. I was looking at my teachers example and I can't understand why the arrows are pointing Left or Right as you know (i assume) directions means a lot since it tells me which is negative and which is positive, so My question is How am I going to know where a certain arrow would be pointing in a certain joint? I have a test tomorrow so can someone help me out? (Is there a counter clockwise-flow or something in it?)
http://www.transtutors.com/homework-help/Mechanical+Engineering/Trusses/method-of-joints-examples.aspx

so what happened here? doesn't it follow the negative (to the left) and positive (to the right)? look @ the FBD for joint A. why is F@AE 'positive' in the summation eq. for x-axis when it is clearly pointing to the left in the FBD?

 

[Robert]

So you have a full diagram, and you've chopped out one joint to analyze. You have the joint itself, and several broken members jutting off in different directions. If an arrow is pointing into the joint, that member is under tension. If the arrow is pointing away from the joint, that member is under compression.

Starting off, you don't know the direction of those arrows or their magnitudes - that's what you're supposed to solve for. Your problem does give you an initial loading, a force or forces acting on the entire frame. Those forces and their directions are what you use to solve for the remaining joints/members. Since it's a static truss, you know all the magnitudes and directions of any joint must sum to 0, or else the system would be in motion.