The work is done by a force is equal to the force multiplied by the distance through which the body has moved in the direction of the force. But if the body is in equilibrium, under the action of a system of forces, the work is done is zero. If we assume that the body in equilibrium, undergoes an infinite small imaginary displacement is known as Virtual displacement. Such an imaginary work is called virtual work.
The concept of virtual work is very useful in finding out the unknown forces in structures. The term ‘virtual‘ is used to stress its purely hypothetical nature, as we do not actually displace the system. We only imagine as to what would happen, if the system is displaced.
Principle of Virtual Work:
It states that “if a system of forces acting on a body or a system of bodies be in equilibrium and the system be imagined to undergo a small displacement consistent with the geometrical conditions, then the algebraic sum of the virtual works done by all the forces of the system is zero“.