1. The problem statement, all variables and given/known data Two identical spaceships are under construction. The constructed length of each spaceship is 1.5 km. After being launched, spaceship A moves away from earth at a constant velocity (speed is 0.850c) with respect to the earth. Spaceship B follows in the same direction at a different constant velocity (speed is 0.500c) with respect to the earth. Determine the length that a passenger on one spaceship measures for the other spaceship. 2. Relevant equations L = Lo (1-v^2/c^2) 3. The attempt at a solution I have: v = v1 - v2 = 0.850c - 0.500c = 0.350c L = Lo (1- v^2/c^2) = 1.5km (1 - 0.350^2) = 1.31km Hi, my question is more whether I'm under-thinking this problem. This solution seems too simple. Should I be using the velocity addition formula or is that only for a third reference frame?