This engineering statics tutorial explains method of joints for truss analysis. You first need to solve for the reaction forces by drawing a FBD of the entire structure. Once you have the reactions, you then need to draw a FBD of every joint. It’s the standard convention to draw unknown forces in tension, so that if you get a positive value for their magnitude, you know you have correctly assumed tension, and if you get a negative value, it just means that the member is in compression. Once you calculate an internal force in a member, it will be a known value for the next joint it touches. So if it’s found to be in compression, then just draw it in compression when you do the next FBD (meaning it’s pushing on the joint rather than pulling on it).
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