A beam is under bending. Let us take a small part of a beam with length x. It has end A and end B. At end A the bending moment is M1 & shear force is S1 and at end B the bending moment is M2 & shear force is S2. Let me take the moment equilibrium for this element about B. M2-S2-M1+S1=0. I need clarity in adding M1 in the above equation. Actually M1 acts with A as the centre. But why this is included while taking moment about B.