Answer:
Work is said to occur when a force is able to cause displacement of an object in the direction of the applied force. It is given by the scalar product of force and displacement.
W = F.s = F s cos θ
Thus, no work is being done when no force is applied ( ∵ F = 0, W = 0) or the force does not result into displacement (∵ s = 0, F = 0) or when the force and displacement are perpendicular to each other (∵ θ =0, cos θ =0, W =0).