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Doppler Effect question

  1. Jun 24, 2009 #1
    Hi, I've got a problem that I can't seem to find an explanation to, about the Doppler Effect

    An ambulance is going down the road at a speed U meters/second, in the same direction as the wind, which has a speed of W meters/second. The speed of sound in air is c meters/second

    Now, I have to choose one of three options:

    1) always c>U+W
    2) always c<U+W
    3) neither (1) nor (2)

    At first I thought that the correct answer was (3), because I couldn't think of a reason why there should be a restriction to the speed of sound in air.
    The correct answer is (1) though, and I can't understand why.
    Can someone please enlighten me?
    Thanks for the help.
     
  2. jcsd
  3. Jun 24, 2009 #2
    I think the reason why it's 1 is because you're assuming that the ambulance isn't breaking the sound barrier. The speed of sound is a constant (it can change due to temperature but we'll assume it's a constant for this problem).

    Also, considering the two other choices, either "c" is less than "U + W" which is obviously false or "c" is equal to "U + W" (assuming it's what is implied) which is also false.
     
  4. Jun 24, 2009 #3
    Is there something conceptually wrong with option 2?
    Let me see if I understand correctly. If the wind has a certain velocity, the resulting velocity of the sound wave is the constant velocity, 343m/s, plus the velocity of the wind. So, is there a reason why option 2 cannot occur?
     
  5. Jun 24, 2009 #4
    I don't think that there is anything wrong with the concept of option 2. But for this problem, I think that they want you to assume reasonable speeds for both the ambulance and the wind.

    For example, option 2 would be correct if the ambulance was going above Mach 1. But how reasonable is that?

    I think it may have something to do with the book the problem came from. Maybe it typically assumes the ordinary.
     
  6. Jun 24, 2009 #5
    Thanks for the explanation!
     
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