Work done is zero in the following cases.
(i) When the force is zero (F = 0). For example, a body moving on a horizontal smooth frictionless surface will continue to do so as no force (not even friction) is acting along the plane. (This is an ideal situation.)
(ii) When the displacement is zero (dr = 0). For example, when force is applied on a rigid wall it does not produce any displacement. Hence, the work done is zero as shown in the figure.
(iii) When the force and displacement are perpendicular (\(\theta\) = 90°) to each other. when a body moves on a horizontal direction, the gravitational force (mg) does not work on the body, since it acts at right angles to the displacement as shown in Figure (b). In the circular motion, the centripetal force does not do work on the object moving on a circle as it is always perpendicular to the displacement as shown in Figure (c).

A compressed or extended spring will transfer its stored potential energy into kinetic energy of the mass attached to the spring. The potential energy-displacement graph is shown in Figure.

In a frictionless environment, the energy gets transferred from kinetic to potential and potential to kinetic repeatedly such that the total energy of the system remains constant. At the mean position,
\(\Delta\)KE=\(\Delta\)U

The unit of power is watt. One watt is defined as the power when one joule of work is done in one second.

The average power is defined as the ratio of the total work done to the total time taken.
P_av = total work done/total time taken
The instantaneous power is defined as the power delivered at an instant
P_inst = dw/dt

In any collision, if the total kinetic energy of the bodies before the collision is equal to the total final kinetic energy of the bodies after collision then it is called an elastic collision. In a collision, the total initial kinetic energy of the bodies before the collision is not equal to the total final kinetic energy of the bodies after the collision. Then it is called an inelastic collision.