Consider this figure. Let us give a virtual displacement 
(downward along inclined path). The work done by R is zero since the motion is perpendicular to it.
The virtual work done by 
The virtual work done by 
By virtua work principle,
This is the same answar which we got earlier using equilibrium equations.We will study more about virtual work later.
| Equation of equilibrium |
Since the particle is in equilibrium, The virtual work done by external active forces on an ideal mechanical system in equilibrium is zero for any and all virtual displacements consistent with the constraints . An eaxmple: |
is zero. The equation 
is therefore an alternative statement of the equilibrium conditions for a particle.