Question

Find the speed at which the mass of an electron is double of its rest mass.

Answer 1

The mass of an electron at speed $v$ is
$m=\frac{m_0}{\sqrt{1-v^2/c^2}}$
where $m_0$ is its rest mass. If $m=2m_0$,
$2=\frac{1}{\sqrt{1-v^2/c^2}}$
or, $1-\frac{v^2}{c^2}=\frac{1}{4}$
or, $v=\frac{\sqrt{3}}{2}c=2.598\times {10}^{8}m{s}^{-1}$.