posted by Katie on .
According to Anna, on Earth, Bob is on a spaceship moving at 0.8c towards Earth, and Carl, a little farther out, is on a spaceship moving a 0.9c towards Earth. According to Bob, how fast is Carl moving relative to Earth?
(a) According to Bob, how fast and in what direction is Carl moving relative to himself (Bob)?
--- I solved part a and got 0.357c and used u' = u-v divide by 1-uv/c^2.
But I confused about part b above.