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Homework Help: Special Relativity - Twins sending light pulses

  1. Sep 5, 2010 #1
    1. The problem statement, all variables and given/known data
    Twin A is on earth. Twin B is aboard a spacecraft that leaves earth toward a start 12LY away at 0.6c.
    Both twins send light pulses every year according to their clocks.
    How many pulses does A send and receive and how many does send and receive if B makes a round trip to the star and back?

    2. Relevant equations

    3. The attempt at a solution

    For twin A I need to know how long it takes B to complete the round trip.

    time = d/r = 24LY/0.6c = 40 years

    Twin a would send 39 pulses since B would arrive at year 40.

    For twin B, the universe is moving past him at 0.6c so I need to compute the length of the trip according to his point of reference.

    L = [tex]L_{0}[/tex][tex]\sqrt{1 - 0.6^2}[/tex] = 9.6LY

    t = d/r = 18.6LY/0.6c = 32 years

    Twin b would send 31 pulses since b would arrive at year 32.
  2. jcsd
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