The capability of doing work is called energy. If something has energy then it can do work. It has the same unit with work joule. Energy in universe can exist in many forms. For example, potential energy in the compressed spring, kinetic energy in the moving object, electromagnetic energy and heat are some of them. In this unit we will deal with mechanical energy of the substances. Mechanical energy is the sum of potential energy and kinetic energy of the system. Let’ see them one by one.