**1. The problem statement, all variables and given/known data**

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?

**2. Relevant equations**

u = (v+u’)/(1+v u’/c^2)

**3. 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)

**1. The problem statement, all variables and given/known data**

**2. Relevant equations**

**3. The attempt at a solution**

http://ift.tt/1uIEDQd