Ball at rest in your reference frame has a mass M. A second ball with the same mass is moving with speed v and collides with ball 1, the balls stick together and continue on the same path that the second ball was on.
A) use conservation of relativistic momentum to find the speed of the combined balls after the collision.
B )find the approximate final speed in the limit of v<
c) find the approximate final speed in the limit of v<=c (roughly equal)
use the equation p=γmv
γ=1/(1-(v^2/c^2))