The

total energy of a

body of mass *m* is given by

**E** = γ*m*c^{2}

where
*total energy* = kinetic energy + rest energy

**or**..

**E** = KE + E_{0}

When a body is at rest γ = 1, KE = 0 and the *rest energy* E_{0} is given by

**E**_{0} = mc^{2}

The rest energy includes all forms of energy internal to the system.

The *kinetic energy* of a body of mass *m* is

**KE** = γmc^{2} - mc^{2}

If the speed of the object is not too large, this reduces too the usual expression

**KE** = ½mv^{2} ( v << c )

Using the expression p=γmv, the total energy of a body can be written as:

**E**^{2} = m^{2}c^{4} + p^{2}c^{2}