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Homework Help: Doppler effect w/ percentage frequency shift?

  1. Dec 2, 2014 #1
    1. The problem statement, all variables and given/known data
    As you stand by a railroad track a train passes with its whistle blowing. As it passes, you hear a frequency shift equal to 21 % of the frequency of the whistle. How fast is the train moving. Express your answer in km/hr.(Speed of sound is 331.4 m/s)

    2. Relevant equations
    f’ = f [1/{1 ± (velocity of source/velocity of sound)}]
    (source moving away from(+)/towards(-) observer)
    3. The attempt at a solution
    My initial attempt was simply to have
    0.79 = [1/(1 + v of train)/331.4] but that seems to be incorrect... a clue from the teacher was that the train is first moving toward, then away from the observer, but I don't understand how that would affect the formula. Any hints? Thanks a ton in advance!
  2. jcsd
  3. Dec 2, 2014 #2


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    2017 Award

    the latter is probably the frequency f0 when the train is standing still. When the train comes at you you hear f' > f0 and when it moves away you hear f'' < f0. Why don't you follow teachers advice and see where it brings you ?
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