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Homework Help: Relativity: Find the magnitude and direction

  1. Oct 1, 2009 #1
    1. The problem statement, all variables and given/known data
    An observer in frame S standing at the origin observes two flashes of colored light separated spatially by Δx = 2400 m. A blue flash occurs first, followed by a red flash 5 μs later. An observer in S' moving along the x-axis at speed v relative to S also observes the flashes 5 μs apart and with a separation of 2400 m, but the red flash is observed first. Find the magnitude and direction of v.

    2. Relevant equations
    v = d/t
    c = 3*10^8

    3. The attempt at a solution
    t2 = time until observer saw blue light
    t1 = time until observer saw red light
    c*t1 + c*t2 = 2400m = Δx (1)
    t1 = t2 + 5μs (2)

    substituting t1 with t2 + 5μs into (1):
    c*(t2 + 5μs) + c*t2 = 2400m
    c*(t2 + t2) = 2400m - 5μs*c
    2*t2 = (2400m - 1498.96229m)/c
    2*t2 = (901.03771m)/c
    t2 = 901.03771m/c/2
    t2 = 1.50276914 microseconds
    t1 = t2 + 5μs
    t1 = 6.50276914 microseconds

    so basically all i found was the time it took the red and blue flash to reach the person. i don't know if i was supposed to do that. i don't really know what to do now so help is very much appreciated.
  2. jcsd
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