Speed of Ships -- Special Relativity

[Barry Melby]

Homework Statement

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?

Homework Equations

The Attempt at a Solution

I thought that the speed would be .550c , but this appears to be incorrect. Any thoughts?

 

[PeroK]

That would be slower than galaxy A is moving.

What do you know about velocity addition?

 

[.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.

 

[PeroK]

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.

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

 

[.Scott]

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

If the direction in which the ship travels is the same as the direction in which A is moving away from B

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