I like to try and derive little things by myself if I think it is manageable. One such thing is the addition of velocities. I gave it a stab, got an answer, and it turned out to be wrong. So tell me where my logic messes up. There's a spaceship moving at velocity u1. A man in the spaceship throws a ball, which from his POV is thrown at velocity u2 in the same direction of the ship. Let's look at the man in the spaceship's POV: In 1 unit of time, the ball will move u2 units of distance. Now, from a man outside of the spaceship, any of distance d0 in the spaceship is observed to have a distance of d0√(1-u12/c2) Also, any event that takes a time of t0 in the spaceship is observed take the time of t0/√(1-u12/c2) So, in the space ship the ball travels d0/t0 But outside the spaceship, I would think that it would take d0/t0 * (1-u12/c2) But that's not it... What's wrong with my reasoning?