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Homework Help: Velocity in Lorentz Transformations

  1. Jan 24, 2009 #1
    I'm reviewing for exams and don't understand when to use which Lorentz velocity equation to use.
    one goes


    and the second

  2. jcsd
  3. Jan 24, 2009 #2


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    You should probably let us know some details such as how your coordinate systems are labeled i.e. is the unprimed system system 'stationary' or moving, and which direction the moving system is going in. Common usage could be assumed, but I'd prefer not to make that assumption.
  4. Jan 24, 2009 #3


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    Homework Helper

    It looks like you have a particle moving at velocity, v, in some frame of reference, K. Then, you consider another frame of reference, K', moving at velocity, u. Then, the equations tell you the velocity of the particle, v', as seen in K' in terms of v, and the velocity of the particle, v, as seen in K in terms of v'.
  5. Jan 24, 2009 #4
    hmm ok here's the question

    two particles move in opposite directions, with one particle at a speed .784c and the other 0.650c as measured by the laboratory. What is the speed of one particle relative to the other.

    To get the right answer it's the second equation= .95c

    v'=.784c u=.650

    the first equation gives .375, which doesn't fit anywhere. So i don't see what it would be used for.
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