It is a fundamental postulate of physics that energy is not created or destroyed. The total energy of the universe is fixed, but it can be transformed from one type of energy to another. In order to make wide use of this principle, many types of energy must be considered (e.g. nuclear, chemical/electrostatic, gravitational, kinetic, etc.). To give a first illustration of the principle, however, introductory physics introduces three energy types that are related to the topics of mechanics: kinetic energy, [gravitational potential energy], and [elastic potential energy]. Collectively, these three types of energy are classified as mechanical energy because of their role in the mechanics of macroscopic bodies.

The mechanical energy (E) of a system is the sum of the system's kinetic and potential energies:

In introductory mechanics, it is basically assumed that the possible constituents of the potential energy are [gravitational potential energy] (U_g)and [elastic potential energy] (U_e), so that the mechanical energy is essentially:

The Work-Kinetic Energy Theorem states:

where W_net is the total work from all sources done on a point particle system.

We can now generalize this theorem slightly to allow for a system composed of rigid bodies that have only mechanical interactions (they do not emit radiation, transfer heat, etc.). We allow for the system to have conservative interactions, but we remove these interactions from the net work and instead account for them using potential energy. Essentially, we move the contribution of conservative forces from the right hand side of the Theorem to the left hand side. The result is:

so that the right hand side is the sum of the works arising from all forces that do not have an associated potential energy.