Astronauts on a long space journey are playing golf inside their spaceship, which is travelling away from the Earth with speed 0.6c. One of the astronauts hits a drive exactly along the length of the spaceship (in its direction of travel) at speed 0.1c in the frame of the spaceship. What is the speed of the gold ball as observed from Earth? ---------- So to the people on earth, the spacecraft itself is obviously going to appear to travel at 0.6c. However since the spacecraft is travelling at relativistic speeds, I thought the speed of the golf ball would not appear to travel at 0.1c to an observer on earth. Instead, length would contract in the direction of motion so it would appear to cover less distance when observed from earth. So the ratio in which the speed should contract would be given by: √(1-(0.6)2) = 0.8 as this is ratio of how much length contracts. And so due to length contraction, the speed of the golf ball would appear to travel at 0.8 x 0.1c. So the ball would go at 0.08c Just adding this to 0.6, the ball would appear to travel at 0.68c. I wasn't to sure if this was right, considering I've never come across a question that involved an object travelling at relativistic speeds within frame of reference that appears to also be moving at a relativistic speed. On top of this, the answer is apparently wrong. The answer says 0.66c but offers no working out. Are the answers wrong or am I missing something?