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Homework Help: The relativity of velocities

  1. Aug 12, 2012 #1
    A particle moves along the x' axis of frame S' with velocity .4c. Frame S' moves with velocity .6c with respect to frame S. What is the velocity of the particle with respect to frame S'?

    I used the realistic velocity equation:
    u=(u' + v)/(1+u'v/c^2)

    u=(.4 + .6)/(1+.4*.6/(3*10^8)^2)

    The answer I get is 1 but the real answer is .81c

    What am I doing wrong?

  2. jcsd
  3. Aug 12, 2012 #2
    You are using u' = .4 and v = .6 instead of u' = .4c = .4*3*10^8 and v = .6c = .6*3*10^8.
  4. Aug 12, 2012 #3
    I see. I keep thinking c is a unit of measurement. It all works now. Thanks!
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