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Homework Help: Doppler effect question

  1. Jan 13, 2009 #1
    1. http://www.prep101.com/MCAT/102MCATPhysicsAnswers.pdf [Broken] number 15

    2. As you can see I already have the answers, I think I'm just doing something wrong.

    3. I already know that the answer will be 'away' making it either a or c. So first I plugged the equation given (f1 = 1.0005f2) to get f2 and got approx 1569.2

    So then I plug the values into this equation Fdelta/ fs = v/c

    Just at the first part of the equation, (subtracting to get delta...I notice that I get .784, which is exactly double the correct answer. What am I missing here? There seems to be no multiplying by .5 or dividing by 2 anywhere in the question. I'm sure it's something small.

    Thanks for the help
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 13, 2009 #2
    Someone correct me if i am wrong, but i suspect the issue is that the medium in which the sound is traveling is also moving. In other words, even though it is the blood that is the source, it is also the medium that the sound is moving in. The result would be that you would have to treat the problem by replacing the source velocity with 2 times the source velocity.
  4. Jan 14, 2009 #3

    Doc Al

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    Staff: Mentor

    It's a bit tricky. You have to apply the Doppler formula twice.

    The flow meter works by sending a signal that reflects off the blood. The received signal is the reflected signal.
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