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## Homework Statement

I know that if you have 2 observers, "A", one at rest and the other "B" moving wrt "A" and if the moving observer shoots a projectile, then we can calculate the velocity of the projectile wrt "A" using the standard equation shown below.

I was thinking what equation would one use in this case: Two spaceships, A and B approach Earth at different velocities wrt the earth. What is the velocity of A wrt B?

## Homework Equations

u = (v+u')/(1+v u'/c^2)

## The Attempt at a Solution

Would u= velocity of A wrt Earth and v= velocity of B wrt Earth and u' be the velocity of A wrt B?

u = (v+u')/(1+v u'/c^2)