Let us consider a differential length dx of the beam shown
In this section, fv will be used for shearing
stress instead of the standard symbol τ.
For the upper shaded portion of the beam, the forces acting are the total normal forces FR and FL due to the bending stresses to the left and to the right of the beam. These forces will be resisted by the shearing force fvb dx acting at the boundary surface between the shaded and the unshaded portions.