Originally Posted by

**acc100jt** In the theory of relativity, the mass of a particle with velocity *v* is

$\displaystyle m=f(v)=\frac{m_{0}}{\sqrt{1-\frac{v^2}{c^2}}}$

where $\displaystyle m_{0}$ is the rest mass of the particle and *c* is the speed of light in vacuum. Find the inverse function of *f*.

I've tried to make *v* the subject and I got this,

$\displaystyle \displaystyle v^{2}=c^{2}\left(1-\frac{m^{2}_{0}}{m^{2}}\right)$

When I square root both sides to get *v*, should I choose the +ve or the -ve square root?

The *v* here is just the speed and no direction is involved here, am I right?