일반물리학 2 — Relativity: Problem 13

13 An armada of spaceships that is 1.00 ly long (as measured in its rest frame) moves with speed 0.850c relative to a ground station in frame S. A messenger travels from the rear of the armada to the front with a speed of 0.950c relative to S. How long does the trip take as measured (a) in the rest frame of the messenger, (b) in the rest frame of the armada, and (c) by an observer in the ground frame S?