The imaginary work done when a system is subjected to infinitesimal hypothetical displacements. According to the principle of virtual work, the total work done by all the forces acting on a system in equilibrium is zero. This principle can be used to determine the forces acting on a system in equilibrium. For example, the illustration shows a ladder leaning against a wall, with the bottom of the ladder attached to the wall by a horizontal weightless string. The tension, T, in the string can be calculated by assuming that infinitesimal movement dx and dy take place as shown. Then by applying the principle of virtual work, Tdx+Wdy=0. As dx and dy can be calculated from the geometry, T can be found.

Virtual work.