Energy can be defined as the capacity to do work. Energy can manifest itself in many forms like mechanical energy, thermal energy, electric energy, chemical energy, light energy, nuclear energy etc. The energy possessed by a body due to its position or due to its motion is called mechanical energy.
The mechanical energy of a body consists of potential energy and kinetic energy.
Potential energy: The potential energy of a body is the energy stored in the body by virtue of its position or the state of strain. Hence water stored in a reservoir, a wound spring, compressed air, stretched rubber chord, etc, possess potential energy.
Potential energy is given by the amount of work done by the force acting on the body. when the body moves from its given position to some other position.
Kinetic energy: The kinetic energy of a body is the energy possessed by the body by virtue of its motion. It is measured by the amount of work that the body can perform against the impressed forces before it comes to rest. A falling body. a bullet fired from a rifle, a swinging pendulum etc. possess kinetic energy.
A body is capable of doing work if it moves, but in the process of doing work its velocity gradually decreases. The amount of work that can be done depends both on the magnitude of the velocity and the mass of the body. A heavy bullet will penetrate a wooden plank deeper than a light bullet of equal size moving with equal velocity.