Stationary particle — UPSC Physics Optional IFoS 2014 Paper I

A particle of rest mass M moving at a velocity u collides with a stationary particle of rest mass m. If the particles stick together, show that the speed of the composite ball is equal to u\alpha M/(\alpha M + m), where \alpha = \frac{1}{\sqrt{1 - \frac{u^2}{c^2}}} .