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Doppler Effect on a Train

  1. Jan 31, 2007 #1
    A train is traveling at 30.0 m/s relative to the ground in still air. The frequency of the note emitted by the train whistle is 262 Hz.
    a. What frequency (fapproach) is heard by a passenger on a train moving at a speed of 18.0 m/s relative to the ground in a direction opposite to the first train and approaching it?

    a. fapproach = (v-vL)/(v+vs) * fs

    f= [(345m/s-18m/s)/(345m/s+30m/s)]*(262 Hz)
    = 228.46 Hz

  2. jcsd
  3. Jan 31, 2007 #2


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    Well, yeah! The frequency of the whistle of a train approaching you is higher, not lower. You have a sign error.
  4. Jan 31, 2007 #3

    Andrew Mason

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    Careful. There are two sets of relative motion here. The source relative to the air and the observer relative to the air. You need to do a doppler calculation for each. As Halls pointed out, the observed frequency should be higher.

  5. Nov 12, 2007 #4
    i dont get it///
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