In the theory of relativity, the mass of a particle with velocity $v$ is $\displaystyle m=\frac{m_0}{\sqrt{1-v^2/c^2}}$, where $m_0$ is the rest mass of the particle and $c$ is the speed of light?
What happens as $v \to c^-$? If necessary, use "i" for infinity.
$m \to$