English: Example illustrating the principle of virtual cut and internal forces and moment.

A cantilever beam supports an uniformly distributed load of intensity q and a concentrated load ${\displaystyle {\vec {\mathrm {B} }}}$ in B. When cutting at a point C, the left trunk (#1) acts on the right trunk (#2) with a force ${\displaystyle {\vec {\mathrm {C} }}_{1/2}}$ and a moment M_C1/2. Reciprocally, the right trunk (#2) acts on the left trunk (#1) with a force ${\displaystyle {\vec {\mathrm {C} }}_{2/1}}$ and a moment M_C2/1;

${\displaystyle {\vec {\mathrm {C} }}_{2/1}=-{\vec {\mathrm {C} }}_{1/2}}$;

${\displaystyle \mathrm {M_{C2/1}} =-\mathrm {M_{C1/2}} }$.

The internal force and moment are either the ones at the left of the cutting plane or the ones at the right.