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Speed of Ships -- Special Relativity

  1. Dec 8, 2015 #1
    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. jcsd
  3. Dec 8, 2015 #2

    PeroK

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    That would be slower than galaxy A is moving.

    What do you know about velocity addition?
     
  4. Dec 8, 2015 #3
    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.
     
  5. Dec 8, 2015 #4

    PeroK

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    I don't think that is right. I believe the ship is moving in the opposite direction from A.
     
  6. Dec 8, 2015 #5
    You're right. So it's simple relativistic addition.
    Just make sure 0.65+0.55<1
     
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