I am studying mechanics of materials and I'm confused with determining internal forces for the sake of stresses.
when analyzing KH cross section, My professors use to draw the free body diagram of the front part (the part that is not attached to the wall.) because you don't worry about reaction forces of the wall in this case.
But I don't understand how they find the direction of reaction forces. (suppose there is no $P_1$ in the diagram and we want to find the reactions of force $P_2$ in cross section KH for the front part (not attached to the wall). just consider the directions and the written labels are just magnitudes of vectors.) which one is correct?:
- they move the force in the way it is, (i.e. if the force is in +x direction, it remains in +x direction) and also the couple caused by that force.
- they move the force but in opposite direction and put a couple in the opposite direction of the couple caused by main force to cancel out all the forces
I would be so thankful if you help me step by step to learn how to do it. I just used this diagram as a sample because didn't find a better one. if you have some better diagrams or so, you are welcome to use it. I just want to learn the easiest method step by step. tell me which part is easier to draw the FBD for? and what steps should I follow? because every time, I get the problem wrong.
thanks in advanced.