## Rest Mass

The rest mass of a particle is the mass a particle has when it's speed is zero, and is labelled

$m_0$
. The mass of a particle is not fixed. As the speed increases, so does the mass, according to the equation
$m=\frac{m_0}{\sqrt{1-v^2/c^2}}$
.

Because
$\gamma = \frac{1}{\sqrt{1-v^2/c^2}} \ge 1$
,
$m_0 \le m$
. .
The rest mass is the most fundamental measure of mass for a particle, but is must be noted that the speed of a particle can never be zero because of the Heisenberg Uncertainty Principle
$\Delta p \delta x \ge \hbar$
. This means that the mass of a particle can never be equal to its rest mass. Nevertheless, the rest mass is an important idea and represents the smallest possible mass of a particle.