Suppose a body is moving with speed **v** in relation to a given reference. As the total energy **AND** of this body is given by:

And the amount of motion (linear momentum) can be obtained by:

It should be emphasized that:

A body with nonzero rest mass cannot reach the speed of light in a vacuum, since if we do **v** tend to **ç**, the mathematical expressions of energy and linear momentum will tend to infinity, which is absurd.

The expression that relates total energy to the amount of motion is given by:

Suppose that the resting mass of a particle is null, the expression above becomes:

So a particle with m** _{0}**= 0 moves with velocity equal to

**ç**. This is the case of photons, which have no resting mass. However, the linear momentum of the photons is not null and, therefore, can be obtained from the equation below: