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What is Virtual work ?

Consider a particle whose static equilibrium position is determined by the three forces which act on it. Any assumed and arbitrary small displacement is called vitual displacement. The work done by any other forces F acting on the particle during the virtual displacement is called virtual work and is,

For an assumed virtual displacement of the particle away from the equilibrium position, the total virtual work done by the particle is,

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Equation of equilibrium

Equilibrium Conditions in terms of Virtual Work:

Equilibrium Conditions can be obtained from virtual work principle also.

The work done by the force F during the displacement d r is defined as,

dU=F.dr

The work done by the couple of magnitude M during the displacement dq is

dU=Mdq