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Homework Help: Doppler Effect detector

  1. Mar 12, 2012 #1
    1. The problem statement, all variables and given/known data
    A detector initially moves at a constant velocity directly toward a stationary sound source and then (after passing it) directly from it. The emitted frequency is f. During the approach the detected frequency is f'app and during the recession it is f'rec. If the frequencies are related by (f'app-f'rec)/f=0.500, what is the ratio v(d)/v of the speed of the detector to the speed of sound?

    2. Relevant equations


    f=frequency emmitted
    f'app=f * [v+v(d)]/v
    f'rec=f * [v-v(d)]/v

    3. The attempt at a solution

    [( f * [v+v(d)]/v)-(f * [v+v(d)]/v)]/f=0.500

    f * ([v+v(d)]/v-[v+v(d)]/v)/f=0.500


    I come up with 1/v=0.500. I guess that would be undefined but it doesn't seem right. I'm not sure where I went wrong here and I've tried a lot of other wierd math techniques to not let v(d) cancel but I can't get it to work out. Any suggestions?
  2. jcsd
  3. Mar 12, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Have you missed out a minus sign in the second term there?

    Note: v+v(d)-v-v(d) = 0, not 1.
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