Einstein's Mass-Velocity Equation

Physical Law
The mass $$m$$ of a body is not constant.

It varies with the body's velocity, according to the equation:
 * $$\displaystyle m = \frac {m_0}{\sqrt{1 - \frac {v^2}{c^2}}}$$

where:
 * $$v$$ is the magnitude of the velocity of the body;
 * $$c$$ is the speed of light;
 * $$m_0$$ is the rest mass of the body.

The value $$m$$ is known as the relativistic mass of the body.

The factor $$\displaystyle \frac 1 {\sqrt{1 - \frac {v^2}{c^2}}}$$ is known as the Lorentz Factor.

Note
Einstein himself suggested in later years that it may be misleading to consider the concept of the relativistic mass. He suggested that it may be better to apply the Lorentz Factor to the momentum instead.