The question is: Which force causes the reaction moment at a fixed joint ?
I understand that at A, there will be a vertical reaction force, a horizontal reaction force and a reaction moment. My understanding is that a moment about an axis is always caused by a force. Which force F causes the reaction moment?
Focusing on this example, suppose F is the horizontal reaction force. From the force equilibrium condition along the x direction, we get that this force has zero magnitude and thus cannot generate the reaction moment.
Suppose F is the vertical reaction force. Apparently, when we cut the beam at some C and apply the moment equilibrium condition at C, we are supposed to include both the reaction moment and the moment generated by the vertical reaction force, which indicates that the moment generated by the vertical reaction force and the reaction moment at A are two different moments caused by different forces.
So, to conclude, if the moment at A is caused neither by the vertical nor the horizontal reaction force, by which force is it caused?