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Homework Help: Special relativity problem

  1. Dec 14, 2011 #1
    1. The problem statement, all variables and given/known data

    Two spaceships (A and B) are travelling in opposite ways. An observer in Earth measures both velocities.
    The speed of ship A (Va) = 0.75c.
    The speed of ship B (Vb) = 0.85c.
    What's the speed of **** B regarding ship A?

    2. Relevant equations

    3. The attempt at a solution

    At first I tried solving it using Galileu's relativity, but I've reached the result of 1.6c.
    Then I remembered that when we're dealing with speeds close to c we have to use Einstein's special relativity, so I've looked around all the formulas I have but none of them involved velocity, only time, spaceship length, and there are also the ones involving energy but I don't think that has nothing to do with this exercise.

  2. jcsd
  3. Dec 14, 2011 #2

    Doc Al

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    Staff: Mentor

  4. Dec 14, 2011 #3
    I've used u'= -.85c and v=.75c

    Substituting I got u= -.85c-.75c / 1-(-.85*.75) = -.977c

    Therefore the speed of ship B regarding ship A is -0.977c

    Is that solution correct? I think I've done everything right but i'd like to be sure :D

  5. Dec 14, 2011 #4

    Doc Al

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    Staff: Mentor

    Good. The relative speed is correct, which is all you need. (The sign depends on your convention.)
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