Static equilibrium is a state in which the net force and net torque acted upon the system is zero. In other words, both linear momentum and angular momentum of the system are conserved. The principle of virtual work states that the virtual work of the applied forces is zero for all virtual movements of the system from static equilibrium. This principle can be generalized such that three dimensional rotations are included: the virtual work of the applied forces and applied moments is zero for all virtual movements of the system from static equilibrium. That is δ δ W = ∑ ∑ i = 1 m F i ⋅ ⋅ δ δ r i + ∑ ∑ j = 1 n M j ⋅ ⋅ δ δ ϕ ϕ j = 0 , where F i , i = 1, 2,..., m and M j , j = 1, 2,..., n are the applied forces and applied moments, respectively, and δ r i , i = 1, 2,..., m and δ φ j, j = 1, 2,..., n are the virtual displacements and virtual rotations, respectively.