The rate of work performed by forces acting on a body is defined by
Principle of Virtual Work
Alternative to the balance equations of linear and angular momentums formulated above, which lead to a boundary value problem, variational principles have been established for describing the motion of a body, leading to integral equations. They are particularly used in numerical methods of continuum mechanics, as there are the finite element or the boundary element method. They are extremum principles for energy type quantities, like work, kinetic energy, potential energy, etc.. Reversely, the differential equations of motion can be established by methods of variational calculus.
