In the theory of relativity, the mass of a particle with speed $v$ is $m=f(v)=\frac{m_0}{\sqrt{1-v^2/c^2}}$, where $m_0$ is the rest mass of the particle and $c$ is the speed of light in a vacuum. Letting $m_0=1$, find $f^{-1}(10)$.

$f^{-1}(10)=$