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Homework Help: Doppler effect

  1. Dec 17, 2016 #1
    1. The problem statement, all variables and given/known data
    An ambulance has a siren of frequency 880 Hz. You are sitting at a stop light as the ambulance passes you, traveling at a constant 22 m/s. If the speed of sound is 343 m/s, what is the change in the frequency that you hear as the ambulance passes you?

    • A

      -53 Hz

    • wrong-icon.png


      -60 Hz

    • C

      -56 Hz

    • correct-icon.png


      -113 Hz

    • E

      -116 Hz
    the correct Answer is D : -113 Hz

    2. Relevant equations
    WHERE FL and vL are the frequency and velocity of sound heard by the personL
    fs and vs are the frequency and velocity of sound emmited by the source

    3. The attempt at a solution
    using the equation we get FL=880x(343+0)/(343-22) therefore FL=940.3Hz
    the difference is FL-Fs=940.3-880=60.311Hz
    but its not correct.
    the correct answer is -113Hz why???
  2. jcsd
  3. Dec 17, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What's the observed frequency when the ambulance is moving towards you? And then away from you?
  4. Dec 17, 2016 #3
    1) when towards me it is FL1=Fs*v/(v-vs)=880*343/(343-22)=940 Hz
    when moves away from me FL2=Fs*v/(v+vs)=880*343/(343+22)=827Hz
    FL2-FL1=827-940=113 Hz
    Ah okay thanksssss
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