Question

Two particles come towards each other with speed 0.9c with respect to laboratory. Calculate their relative speeds.

Answer 1

The relative velocity

$$v_{r}=\frac{v+u}{1+\frac{vu}{c^2}}$$

$$v_r=\frac{0.9c+0.9c}{1+\frac{(0.9c)^2}{c^2}}=0.994c$$