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## Homework Statement

- 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

## Homework Equations

## 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-u

^{2}/c

^{2}= 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 V

_{y}' = V

_{y}/γ(1 - uV

_{x}/c

^{2}where the dash frame is the frame of the ship. For this question would i take V

_{x}= 0 and V

_{y}= 0.85c and just end up with V

_{y}' = V

_{y}/γ? There would then be only 'north' velocity and none in the east direction?