Law of conservation of Energy

The law of conservation of Energy states that the energy can neither be created nor destroyed, though it can be transformed from one form to another form, in which the energy can exist.

Assume a block of mass m falls from rest from a height H.

This shows that the total energy of the ball remains constant anywhere in between A to C. Thus, K.E. and P.E. individually change throughout the motion from A to C, but their sum always remains constant.

Work – Energy Relation

According to the work-energy theorem, the work done by an external force on a moving body is equal to the change in kinetic energy of the body.
Suppose an external constant force F is applied on a body of mass m and due to this force velocity of mass changes from u to v.

According to this theorem, if work is done on the body by applying an external force, then it increases the kinetic energy of the body and if a body does work against retarding force (frictional force), then its kinetic energy decreases. If there is no change in the velocity of the body with application of force, then there is no change in kinetic energy. So work done by the force will be zero.