Definition of energy.

Measure of the ability of a body or system to do work or produce a change, expressed usually in joules or kilowatt hours.No activity is possible without energy and its total amount in the universe is fixed. In others words , it cannot be created or destroyed but can only be changed from one type to another. The two basic types of energy are(1) Potential : energy associated with the nature , position , or state (such as chemical energy , electrical energy nuclear energy. ).(2)Kinetic : energy associated with motion (such as a moving car or a spinning wheel).