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Homework Help: The Doppler Effect on railroad tracks

  1. Nov 12, 2008 #1
    1. The problem statement, all variables and given/known data

    Do not attempt to do this experiment! You are sitting on railroad tracks,
    and extremely fast trains are approaching from both the left and the right.
    These trains have equal speeds, and both send out a warning signal with their
    horns. You hear this signal at a frequency which is 50% higher than the emitted
    frequency. Did I say that these trains went fast? How large is the change in
    frequency by the signal sent out by one train and observed by the machinist in
    the other?

    2. Relevant equations

    f+=f0/1-Vs/V (approaching source)
    f-=f0/1+Vs/V) (receding source)
    f+=(1+V0/V)f0 (observer approaching a source)
    f-=1(-V0/V)f0 ( observer receding from a source)

    3. The attempt at a solution
    I just need a little help getting started, I don't know where to go with this.
  2. jcsd
  3. Nov 12, 2008 #2
    In your case, you are observing the sound from a source approaching you. So, use the appropriate formula, and infer what the speed of the trains (the source) must be.

    Then you have to use that information to figure out what the Doppler effect amounts to for the case where both observer and source are moving.
  4. Nov 12, 2008 #3
    thanks! i think i got it.
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