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Homework Help: Doppler Effect: Velocity of a train

  1. Jun 29, 2017 #1
    1. The problem statement, all variables and given/known data
    To determine the speed of a train, a student of Physics determined the frequency of the whistle in the approach to its position of observation, and measured 220 Hz. He then determined the frequency of the whistle when the train moved away, and got 190 Hz. What is the speed of train? Consider that the velocity of sound in the air is 340 ms-1

    2. Relevant equations

    Doppler effect equation:
    $$f' = f \frac{u+- v_Dt}{u+- v_St}$$

    3. The attempt at a solution
    What I simply did was to isolate v_D to determine it.

    $$v_D=v(1-\frac{f'}{f})= 340(1+\frac{190}{200}=46.4$$

    Note: I choose the minus signal in the numerator because the destiny is moving away from the source.

    The answer of textbook is however:
    $$v_D=v(\frac{f-f'}{f+f'})= 24.9 m/s$$

    I don't get what I'm doing wrong. Can someone help me?
  2. jcsd
  3. Jun 29, 2017 #2
    Maybe I'm crazy but this doppler equation doesn't feel right. Why is there a time dependence? The frequency should not be changing as a function of time. Check this.

    In any case, you solved for vD. Is this the train? Which object is the source of the sound, the train or the student?
  4. Jun 29, 2017 #3
    Hey! You are right there is no time dependency (note that I didn't use it when doing the calculation). Sorry!

    The way I interpreted the problem is that the student is the source and the train is the detector.
  5. Jun 29, 2017 #4
  6. Jun 29, 2017 #5


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    Taking the time dependence out of your Doppler Effect Formula (I also assumed that you mean ±) gives:
    ##\displaystyle f' = f\, \frac{u\pm v_D}{u\pm v_S} ##​

    Of course, the first thing to do is to give the definitions all of those quantities.
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