1. The problem statement, all variables and given/known data Two light beams collide head on. Calculate their relative velocity. (c) A particle moves north at speed 0.85c relative to an observer standing on the Earth. What is the velocity of this particle as observed by a fast ship traveling east on the Earth at speed 0.9c? Give the direction of travel with respect to a compass direction 2. Relevant equations 3. The attempt at a solution 1) A little confused by this question. I thought that speed of light is a constant no matter what reference frame it is viewed from so their relative velocity would be just c? 2) So speed between inertial frames u = 0.9c. Gamma γ = 1/√1-u2/c2 = 1/√0.19 However i'm unfamiliar with working with the velocity of the object perpendicular to the relative velocity between the frames. I've found Vy' = Vy/γ(1 - uVx/c2 where the dash frame is the frame of the ship. For this question would i take Vx = 0 and Vy = 0.85c and just end up with Vy' = Vy/γ? There would then be only 'north' velocity and none in the east direction?