Moment-beam under bending momen?

[chandran]

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.

 

[Astronuc]

M is a moment or torque, which is not a force, but the effect of a force applied at some moment arm.

M has units of F-d (force - distance, e.g. N-m or ft-lbm).

In a static beam situation, the sum of the forces must = 0, and the sum of the moments = 0.

Here is a good reference on beam mechanics (statics) -
http://physics.uwstout.edu/StatStr/statics/Beams/beam41.htm#Topic 4.1 Shear Forces I

and

http://physics.uwstout.edu/StatStr/statics/