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GRE Doppler Effect Question

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data
    http://grephysics.net/ans/8677/12

    I don't understand how it increases it's frequency by a factor of 10, I mean the total velocity goes from 1 to 1.9, therefore It seems like it should increase from 1kHz to 1.9kHz. Why is my 'common sense' wrong?
     
  2. jcsd
  3. Sep 2, 2013 #2

    Doc Al

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    Staff: Mentor

    What does the Doppler formula give you for the observed frequency when the source is moving? Not sure what you mean by "total velocity"--realize that the source is moving towards the listener.

    (I misread the problem myself, the first time through.)
     
  4. Sep 2, 2013 #3
    frequency receiver = (velocity sound + velocity sender) * frequency sender
    with velocity of sound = c and velocity of sender = .9c, It seems the frequency received is 1.9 the original frequency. No?
     
  5. Sep 2, 2013 #4

    Doc Al

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    Staff: Mentor

    No.

    The Doppler formula for an approaching source is:
    ##f' = \frac{v}{v - v_s} f##
    where ##v_s## is the speed of the source.

    See: Doppler Effect
     
  6. Sep 2, 2013 #5
    Okay so the correct Doppler effect the source is in the denominator, I don't understand qualitatively why it matters though. Whether it's the receiver or sender that's movie, the relative velocity between them is going to be the same?
     
  7. Sep 2, 2013 #6

    Doc Al

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    Staff: Mentor

    Note that relative velocity (of source and observer) doesn't enter into the Doppler formula (at least for sound). The speeds are relative to the air. And that a moving source differs from a moving observer. (Compare the two cases, even for the same speed.)

    To understand how this is, you'll need to review the details of the derivation of the formula.
     
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