In simply supported beams, before going for shear force and bending moment calculations, the reactions (RA and RB) at the supports A and B should be determined.
To determine the reactions, moment of all forces with respect to one support is taken and equated to zero. By solving this equation, one reaction can be found. The other reaction is determined by equating upward forces and the downward forces.
Shear force and bending moment diagrams are determined as done in cantilever beams.
Let AB be the simply supported beam carrying a concentrated load W at the mid-span (at point C).
As the load is symmetrically placed ...