1. The problem statement, all variables and given/known data A train with proper length L moves at speed 5c/13 with respect to the ground. A ball is thrown from the back of the train to the front. The speed of the ball with respect to the train is c/3. As viewed by someone on the ground, how much time does the ball spend in the air and how far does it travel. 2. Relevant equations Lorentz velocity transformation v(x)= (v(x)'+u)/(1+uv(x)'/c^2) Length Contraction (?) 3. The attempt at a solution First I tried to find the speed of the ball with regards to the person on the ground. Using the Lorentz velocity transformation, with v(x)' as c/3 and u as 5c/13, I got v(x)=7c/11 After that I'm not sure about the next step. The answer to the first part of the problem is 11L/3c. However, when I assumed that the ball would need to travel L distance more than the train to get from back to front, I get L/(7c/11-5c/13)=143L/36c. I'm just really confused now. I think I can solve the second part of the question if I figure out what's wrong with my approach to the first. Thank you!