I am looking over an old problem about a cart that has a pendulum on it, and you are supposed to find the equations of motion. The pendulum is made from a mass and a wire. Because wire can only support tensile loads, the tension has to be directed along the direction of the wire. Any force not directed along the axis of the wire will cause it to buckle. But consider I replace the wire with a rigid bar. So I now have a rigid pendulum. I want to say that the force in the bar will remain along the axis of the bar, but I cant for the life of me show why. Because it is now a bar, it has to be pin connected to the cart, which means it can only have a reaction force in the (x,y) direction at the pin. Any ideas?