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Homework Help: Waves and Sound -

  1. Dec 9, 2013 #1
    1. The problem statement, all variables and given/known data

    Two train whistles have frequencies of 180 Hz in air (v = 344 m/s). When one train is at rest sounding its whistle, a beat frequency of 2 Hz is heard from the moving train which is sounding its whistle as well. What is the moving train's speed if it is approaching the station?

    2. Relevant equations

    fbeat = l fo - f1 l

    fo = fs (v + vo / v - vs) ; [source is approaching the observer]

    fo: observer
    fs: source

    3. The attempt at a solution

    v = 344 m/s
    fs = 180 Hz
    fo = 182 Hz [fo = fbeat + fs; = 2 Hz + 180 Hz]
    vo = 0
    vs = ?

    fo = fs (v + vo / v - vs) ; [source is approaching the observer]

    I'm trying to rearrange this formula to isolate vs but for some reason I can't.
    Where do I start... I've tried everything. :confused:

    And sorry about all the questions, I'm having some significant trouble with this unit in physics...
  2. jcsd
  3. Dec 9, 2013 #2
    The first step is to multiply both sides by (v - vs). Can you see why? What is the next step?
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