A shear-force diagram and bending-moment diagram show how the internal shear $V$ and bending moment $M$ vary along a beam.

Start from the external loads

Draw the beam's free-body diagram and determine its support reactions.

Then consider cuts at successive positions along the beam. For each interval between concentrated loads or applied moments, use static equilibrium of one side of the cut to determine the internal shear force and bending moment.

Build the diagrams

Plot the calculated values against position along the beam. Concentrated transverse forces produce jumps in the shear diagram, while concentrated applied moments produce jumps in the bending-moment diagram.

Distributed loads make the internal actions vary continuously across the loaded region.

Why the diagrams matter

The diagrams make it easy to identify where the internal shear force and bending moment are largest. Those critical regions often control the beam's stress and deformation.