In the special case of elastic bodies, the right-hand-side of (3) can be taken to be the change, δ δ U , of elastic strain energy U due to infinitesimal variations of real displacements. In addition, when the external forces are conservative forces, the left-hand-side of (3) can be seen as the change in the potential energy function V of the forces. The function V is defined as: V = − − ∫ ∫ S t u T T d S − − ∫ ∫ V u T f d V where the minus sign implies a loss of potential energy as the force is displaced in its direction. With these two subsidiary conditions, Equation 3 becomes: − − δ δ V = δ δ U This leads to (2) as desired. The variational form of (2) is often used as the basis for developing the finite element method in structural mechanics.