Computation of Work Done on a Body:

Assume a body of weight W moving on a horizontal plane through a distance AA′ = d under the action of force F applied along a direction at an angle θ with the horizontal.

Although the distance travelled by body along the horizontal plane of movement is d = AA′ = BB′ , work done by force F is given by (F × s) where s is the component of the displacement d along direction of F.

                                           s = d cos θ

                              ∴ Work done = F × d cos θ = A function of ∠ θ

If,       ∠ θ → 90^o,             W. D. → 0.

From the above discussion, you shall realize that gravitational force on a body will not do any work, if the body moves in a horizontal plane.

Consider another example of a body moving down an inclined plane of distance (d) where inclination of plane with the horizontal is ∠ α, work done by the weight W of the body is given by

W. D. = W d sin α

∴ Vertical component of distance travelled = d sin α