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Doppler Effect?

  1. May 18, 2017 #1
    1. The problem statement, all variables and given/known data
    You are standing on a train station platform as a train goes by close to you. As the train approaches, you hear the whistle sound at a frequency of f1 = 92 Hz. As the train recedes, you hear the whistle sound at a frequency of f2 = 79 Hz. Take the speed of sound in air to be v = 340 m/s.

    Find the numeric value, in hertz, for the frequency of the train whistle fs that you would hear if the train were not moving.

    2. Relevant equations
    f obs = f s (v +- vobs / v +- vs)

    3. The attempt at a solution
    I tried to find fs by fs = f obs / (v +- vobs / v +- vs), but since nobody is moving I just get 1 in parenthesis, and don't know what to set f obs to. The correct answer is 85 Hz, I just have no clue how to get to that point.
     
  2. jcsd
  3. May 18, 2017 #2

    kuruman

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    Isn't the train moving?
     
  4. May 19, 2017 #3
    Well it says to find the numeric value, in hertz, for the frequency of the train whistle fs that you would hear if the train were not moving.
     
  5. May 19, 2017 #4

    kuruman

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    You quoted the relevant equation
    $$f_{obs}=f\frac{v \pm v_{obs}}{v \pm v_{s}}$$
    Can you identify what these symbols stand for? For example,
    v = speed of sound, here 340 m/s.
    What about ##f##, ##f_{obs}##, ##v_{obs}## and ##v_s##? Can you say with words what they stand for and, if known, what their values are?
     
  6. May 19, 2017 #5
    f is the frequency produced from the source.
    fobs is the frequency heard by the observer.
    Vobs is the velocity of the observer (standing still so it has to be 0).
    Vs is the velocity of the source, which in this part of the problem is the train which we are told stopped moving.

    The only thing we have are the frequencies heard by the observer (92 and 79) along with the velocity of sound 340 m/s
     
  7. May 19, 2017 #6

    haruspex

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    But those data about the two frequencies heard apply to the case where the train is moving.
     
  8. May 19, 2017 #7

    kuruman

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    Indeed, and that is what the problem is asking you to find. What do you think given frequencies f1 = 92 Hz and f2 = 79 Hz are? Where do they come from and under what circumstances? If the train is not moving, the Doppler formula gives ##f_{obs} = f##; that's nothing new. As @haruspex hinted, what does the formula look like when the train is moving?
     
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