1. The problem statement, all variables and given/known data A starship is moving toward a space station at half the speed of light. When it is 7.00 seconds away from reaching a space station (as measured on the ship's clocks) the starship fires a projectile toward the space station at 0.600 c (as measured by the ship's crew). When will the projectile strike the space station, as measured by the ship's clocks and also by the space station's clocks? Correct answers: -3.82 s and 3.31 s, respectively. 2. Relevant equations delta t = gamma*delta t_proper d = rt 3. The attempt at a solution I tried solving this problem from the starship's point of view. From the starship's point of view, it is stationary, the station is moving 0.5c toward it, and the projectile is moving 0.6c toward the station. The distance between the ship and the station @ t = -7 s can be found by d = rt = (0.5c)(7 s) = 3.5 Ls Then the time it takes for the projectile to hit the projectile to collide with the station is t = d / r = 3.5 Ls / (0.5c+0.6c) = 3.18s. But the ship's clocks started at t = -7 s, so the time read on the clocks as the projectile hits the station is t = -7 s + 3.18 s = -3.82 s, which is the correct answer. Now I have a few questions about this: Why do we say that the time at which the projectile collides with the station is t = 0? I'm having an awful time figuring out what is the proper time and what is the... regular time? Which is which and why? Could this problem be solved from the perspective of the space station? If so, how? Finally, how can I solve this problem by using a space time diagram like this one? http://i.imgur.com/BJ0tbXs.jpg?1 What goes on the x-ct axis and what goes on the x'-ct' axis?