(a) the force is always perpendicular to its velocity
(c) the object is stationary but the point of application of the force moves on the object
(d) the object moves in such a way that the point of application of the force remains fixed.
Explanation:
Since there is no displacement along the force when it is perpendicular to the velocity of an object, so no work is done. Answer (a). (b) is not true because the acceleration will always be in the direction of force according to Newton's Second Law of Motion.
No work is done if there is no displacement of the object in the direction of the force. Answer (c).
Again there is no displacement along the force, no work done. Answer (d).
