Editing Virtual work (section)

===Principle of virtual forces===
Here, we specify:

* Virtual forces and stresses as variations of the real forces and stresses.
* Virtual forces be zero on the part <math> S_t </math> of the surface that has prescribed forces, and thus only surface (reaction) forces on <math> S_u </math> (where displacements are prescribed) would do work.

The virtual work equation becomes the principle of virtual forces:
{{NumBlk||<math display="block"> \int_{S_u} \mathbf{u}^T \delta\ \mathbf{T} dS + \int_{V} \mathbf{u}^T \delta\ \mathbf{f} dV = \int_{V} \boldsymbol{\epsilon}^T \delta \boldsymbol{\sigma} dV </math>|{{EquationRef|g}}}}

This relation is equivalent to the set of strain-compatibility equations as well as of the displacement boundary conditions on the part <math> S_u </math>. It has another name: the principle of complementary virtual work.