A mechanical force analysis tripod, rod force Does not mean that the force on the rod is not necessarily along the rod? Why do we have to do force analysis along the rod?

A mechanical force analysis tripod, rod force Does not mean that the force on the rod is not necessarily along the rod? Why do we have to do force analysis along the rod?

The bar in this problem is the so-called two force bar (only under tension or pressure). The direction of force on the two force bar must be along the bar
Your question is: the force on the rod is not necessarily along the rod, which is true. For the rod with bending moment, the force is not along the rod
Most of the physics problems in your high school are two bar, and there is almost no bending moment or torque on the bar!