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Homework Help: Special Relativity - Velocity transformation problem

  1. Sep 11, 2012 #1
    1. The problem statement, all variables and given/known data

    Two spaceships are moving away from Earth at a speed of 0.8c, with one ship following in the flight path of the other. Their separation along the axis of their motion is maintained at 0.1 light years as measures by the spaceships' instruments. A crew exchange vehicle is launched from the rear spaceship to the front spaceship, travelling at 0.9c relative to Earth. Approximately, how long does the trip take for the crew inside the crew exchange vehicle.

    2. Relevant equations

    Lorentz Velocity Transformations:

    u' = [itex]\frac{u-v}{1-\frac{vu}{c^{2}}}[/itex]

    3. The attempt at a solution

    u' = [itex]\frac{0.9c-0.8c}{1-0.72}[/itex]
    = 0.357c


    L' = [itex]\sqrt{1-\frac{v^2}{c^2}}[/itex]L[itex]_{0}[/itex]
    = 0.093 light years

    [itex]\frac{0.093}{0.357}[/itex] = 0.26 years, or 95 days.

    However, the question is multiple choice, and the options are:

    A. 1 year
    B. 25 days
    C. 1 day
    D. 10 days
    E. 150 days

    Can someone please point out where I might have slipped up. Thankyou!
  2. jcsd
  3. Sep 11, 2012 #2
    Edit: strike that, I think you did the velocity transformation all right.

    What did you plug in for [itex]v[/itex] in the length contraction equation?
  4. Sep 11, 2012 #3
    0.357c. And v isn't great enough for the length contraction to be the part I messed up on.
  5. Sep 11, 2012 #4
    Hm, okay, I worked the problem just in Earth's reference frame and got a different answer, but the calculation is sufficiently involved that I'm not entirely sure on it. Are you familiar with trying to find the point of intersection of two lines? Doing that for the worldlines of the ship and the exchange vehicle may give another useful check.
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