Internal forces are the forces in the connection between members. During any positive movement of the system, the net work done by the internal forces at the connection is zero. This is because the internal forces always exists in pairs of equal and opposite forces.

Virtual Work Method

Active forces are external forces capable of doing virtual work during positive virtual displacements. In the above truss, P is the active force.

Reactive forces are forces which act at fixed support positions where no virtual displacement takes place in the direction of the force. Reactive forces do not do work during a virtual displacement. Figure below shows the reactive forces R₁ and R₂ on the truss.