# Speed of Ships -- Special Relativity

1. Dec 8, 2015

### Barry Melby

1. The problem statement, all variables and given/known data
Galaxy A moves away from galaxy B at 0.650 c0 relative to B. A spaceship leaves a planet in galaxy A traveling at 0.550 c0 relative to galaxy A. If the direction in which the ship travels is the same as the direction in which A is moving away from B, what do observers in B measure for the ship's speed?

2. Relevant equations

3. The attempt at a solution
I thought that the speed would be .550c , but this appears to be incorrect. Any thoughts?

2. Dec 8, 2015

### PeroK

That would be slower than galaxy A is moving.

3. Dec 8, 2015

### .Scott

From B's point of view, A is moving away at 0.65c. Since the spaceship is moving away from A in the direction of B at 0.55c, B will see the ship move away - but not as fast as 0.65c.

4. Dec 8, 2015

### PeroK

I don't think that is right. I believe the ship is moving in the opposite direction from A.

5. Dec 8, 2015

### .Scott

You're right. So it's simple relativistic addition.
Just make sure 0.65+0.55<1