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Doppler Effect Question

  1. Jan 18, 2005 #1
    In the doppler effect equation for sound, measuring frequency, I have the following question:

    Car 1-20m/s--> Car 2-30m/s-->
    --POINT A--


    With the above "diagram", we have car 1 going at 20 m/s, right, and car two, 30m/s right, with respect to point A (doesn't have to be those values, just an example).

    If we were to look at car 2, relative to car 1, car 2 would be moving away from car 1 at 10m/s. However, when we plug both the raw values, and the relative values into the equation, they yield a different frequency. Why is this? :|
     
  2. jcsd
  3. Jan 18, 2005 #2

    Andrew Mason

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    I think you have to take into account the speed of the source relative to the medium as well as the observer. Have a look at this:
    http://usna.edu/Users/physics/mungan/Scholarship/DopplerEffect.pdf

    AM
     
  4. Jan 18, 2005 #3
    Yes, I'm aware of that. However, why would that be different? In classic motion scenarios, it renders the same answer, does it not? Since sound isn't in any special realms of physics, why is this different?
     
  5. Jan 18, 2005 #4
    Well, you would be hearing a different sound while you are in Car 1 and while you are on the ground, would you not?
     
  6. Jan 19, 2005 #5

    Andrew Mason

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    The cite I gave you should explain it for you. Think about the case in which the car is travelling at the speed of sound. Does the sound every reach an observer at rest with respect to the medium positioned behind the car? How about in front? So the motion relative to the medium is important.

    AM
     
  7. Jan 19, 2005 #6
    The site doesn't work =(
    I understand your example, I'm still wondering why that is, though. I'll try and figure it out :D
     
  8. Jan 19, 2005 #7

    Andrew Mason

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    Last edited: Jan 20, 2005
  9. Jan 20, 2005 #8
    I have that installed, and I can read pdfs :P The link simply does not work for me =\
     
  10. Jan 20, 2005 #9

    Andrew Mason

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    You must be copying it and pasting it into your browser. That won't work. You have to click it

    AM
     
  11. Jan 23, 2005 #10
    I am clicking it. I know how to operate a computer... ... =x
     
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