(a) Work done is zero (i) when there is no displacement of the object. (ii) When the angle between force and the displacement is 90º. When a mass moves on a horizontal plane the work done by gravitation force is zero
(b) When a particle is thrown verically upwards, the work done by gravitational force is negative.
(c) When a particle moves in the direction of force, the work done by force is positive.
