Definition of energy: Energy is the ability of a body to do work.
Step-by-step explanation:
Work and energy are inter convertible in nature. When work is done on the body or by the body, some amount of energy will be utilized in that process. On the other hand, we may say that work is done in the expense of energy.
Hence, energy of a substance can be defined as the capacity of that substance to do work.
For instance, we may take an example of rubbing of hands. When we rub our two hands together, we do some amount of work. That work is equivalent to the heat produced in our hands.
When a body moves with certain velocity , it gains some kinetic energy. The work done by that body is nothing else than the change in kinetic energy of the body.