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Homework Help: Relativistic Velocity Addition

  1. Feb 11, 2010 #1
    Nevermind, got it.

    Two spaceships approach the Earth from opposite directions. According to an observer on the Earth, ship A is moving at a speed of 0.753c and ship B at a speed of 0.851c. What is the speed of ship A as observed from ship B? Of ship B as observed from ship A?

    2. Relevant equations
    v = (v' + u) / (1+ (v'u)/(c^2))
    and so
    v' = (u - v) / ((vu)/(c^2) - 1)

    3. The attempt at a solution
    u is the speed of A as observed from Earth.
    v is the speed of B as observed from Earth.
    v', then, would be the speed of A or B as observed from the other ship.

    The values 0.753c and 0.851c plugged in, I get 0.2728c = v'.

    The speed of one ship as observed by the other ship should be greater than either of the ship's speeds as observed on earth. So where am I incorrect?
    Last edited: Feb 11, 2010
  2. jcsd
  3. Feb 11, 2010 #2
    Nevermind, got it just after I posted. I needed a negative! Always a negative.