### Variation of Mass with Velocity in Relativity

Derivation of variation of mass with velocity:
Consider two systems of reference (frame of reference) $S$ and $S’$. The frame $S’$ is moving with constant velocity $v$ relative to frame $S$.

Let two bodies of masses $m_{1}$ and $m_{2}$ be traveling with velocities $u’$ and $-u’$ parallel to the x-axis in the system $S’$. Suppose the two bodies collide and after collision coalesce into one body.
The principles of conservation of mass and of momentum also hold good in relativity same as in classical mechanics. So now apply the principle of conservation of momentum.

$m_{1}u_{1}+m_{2}u_{2}=\left ( m_{1}+m_{2} \right )v\qquad(1)$

Apply the law of addition of velocities, the velocities $u_{1}$ and $u_{1}$ in the system $S$ corresponding to $u’$ and $-u’$ in frame $S’$ are given by $\rightarrow$

$u_{1}= \frac{u'+v}{1+\frac{u'v}{c^{2}}}\quad or \quad u_{2}= \frac{-u'+v}{1-\frac{u'v}{c^{2}}}\qquad(2)$

Now substitute the value of $u_{1}$ and $u_{1}$ in equation $(1)$

$ m_{1}\frac{u'+v}{\left ( 1+\frac{u'v}{c^{2}} \right )}+m_{2}\frac{-u'+v}{\left ( 1-\frac{u'v}{c^{2}} \right )}=\left ( m_{1}+m_{2} \right )v $

$ m_{1}\left [ \frac{u'+v}{1+\frac{u'v}{c^{2}}}-v \right ]=m_{2}\left [ v- \frac{-u'+v}{1-\frac{u'v}{c^{2}}} \right ]$

$ m_{1}\left [ \frac{u'+v-v-u'\frac{v^{2}}{c^{2}}}{1+u'\frac{v}{c^{2}}} \right ]=m_{2}\left [ \frac{v-u'\frac{v^{2}}{c^{2}}+u'-v}{1- u'\frac{v}{c^{2}}} \right ]$

$ \frac{m_{1}}{m_{2}}= \frac{\left ( 1+u'\frac{v}{c^{2}} \right )}{\left ( 1-u'\frac{v}{c^{2}} \right )}\qquad(3)$

From equation$(2)$

$u_{1}^{2} =\left ( \frac{u'+v}{1+\frac{u'v}{c^{2}}} \right )^{2}$

$ 1-\frac{u_{1}^{2}}{c^{2}}=1- \frac{1}{c^{2}} \left ( \frac{u'+v}{1+\frac{u'v}{c^{2}}} \right )^{2}$

$ 1-\frac{u_{1}^{2}}{c^{2}}= \frac{\left ( 1+\frac{u'v}{c^{2}} \right )^{2}-\left ( \frac{u'+v}{c} \right )^{2}}{\left ( 1+\frac{u'v}{c^{2}} \right )^{2}}$

$ 1-\frac{u_{1}^{2}}{c^{2}}= \frac{\left ( 1-\frac{u'^{2}}{c^{2}} \right )\left ( 1-\frac{v^{2}}{c^{2}} \right )}{\left ( 1+\frac{u'v}{c{2}} \right )}$

$ \left ( 1+\frac{u'v}{c^{2}} \right )^{2}=\frac{\left ( 1-\frac{u'^{2}}{c^{2}} \right )\left ( 1-\frac{v^{2}}{c^{2}} \right )}{1-\frac{u_{1}^{2}}{c^{2}}}$

$ \left ( 1+\frac{u'v}{c^{2}} \right ) =\left [ \frac{\left ( 1-\frac{u'^{2}}{c^{2}} \right )\left ( 1-\frac{v^{2}}{c^{2}} \right )}{1-\frac{u_{1}^{2}}{c^{2}}} \right ]^\frac{1}{2}\qquad(4)$

Similarly, square the velocity of $u_{2}$ and solve the as above, so

$\left ( 1-\frac{u'v}{c^{2}} \right ) =\left [ \frac{\left ( 1-\frac{u'^{2}}{c^{2}} \right )\left ( 1-\frac{v^{2}}{c^{2}} \right )}{1-\frac{u_{2}^{2}}{c^{2}}} \right ]\qquad(5)$

Now substituting the values from equation $(4)$ and equation $(5)$ in equation $(3)$

$\frac{m_{1}}{m_{2}}=\frac{\left ( 1-\frac{u_{2}^{2}}{c^{2}} \right )^{\frac{1}{2}}}{\left ( 1-\frac{u_{1}^{2}}{c^{2}} \right )^{\frac{1}{2}}}$

If the body of mass $m_{2}$ is at rest i.e. $m_{2}=m_{0}$ so velocity of the body in frame-S will be zero. i.e. $u_{2}=0$

Where $m_{0}$ is the rest mass.

$m_{1}=\frac{m_{0}}{\left ( 1-\frac{u_{1}^{2}}{c_{2}} \right )^{\frac{1}{2}}}$

Let $ m_{1}=m$ and $u_{1}=v$ so above equation

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

This is the generalized formula of variation of mass with velocity.