Energy is the capacity of a body to work. Energy can exist in various forms which are mutually convertible. Since the energy is measured in terms of work, therefore, its units are same as that of work.
Basically there are two types of energy namely potential energy and kinetic energy.
The work done by a body by virtue of its motion is called its kinetic energy (KE). It is measured by the work it can do against an opposing force before it is brought to rest.
Potential energy is the energy possessed by a body due to its position with respect to another body. The potential energy of body with respect to the earth is measured by amount of work in lifting it through a certain vertical height.
Definition of conservation and transfer of energy
Energy can neither be created nor be destroyed, it can only be converted from one form to another. In other words, the net energy in the universe always remains constant. This is called principal of conservation of energy.
Transfer of energy can be referred to as when the energy gets converted from one form to another or from one body to another body we say that transfer of energy is taking place between bodies.
Examples on conservation and transfer of energy
1) When a body is projected vertically upwards, its velocity goes on decreasing till the velocity is reduced to zero. The initial kinetic energy of the body decreases and it changes into potential energy such that at any instant the sum of the kinetic energy and potential energy is a constant. This constant is equal to the initial kinetic energy of the body comes to rest at the highest point.
2) When the bob of a simple pendulum is displaced from its equilibrium position and then it is released, then its initial potential energy changes into kinetic energy into the vertical position. As the bob travels to the other extreme position the kinetic energy changes into potential energy. These changes in energy continuously take place as the bob swings between the two extreme positions, such that the sum of the kinetic energy and potential energy is always the same.