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Homework Help: Doppler effect moving source

  1. Apr 14, 2017 #1
    1. The problem statement, all variables and given/known data
    A stationary police car emits a sound of frequency 1200 Hz that bounces off a car on the highway and returns with a frequency of 1250 Hz. The police car is right next to the highway, so the moving car is traveling directly toward or away from it. (a) How fast was the moving car going? Was it moving toward or away from the police car?

    2. Relevant equations
    ƒL = ƒS(v - vL)/(v - vS)

    3. The attempt at a solution
    The returning frequence is larger so its wave length has to be smaller which tells us that the car is moving towards the police car.

    vS = v - vƒSL = 13.76 m/s
    The correct answer is 7.02 m/s.
    What am I doing wrong?
  2. jcsd
  3. Apr 14, 2017 #2

    Doc Al

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    Staff: Mentor

    Hint: You need to apply the Doppler formula twice.

    1) What frequency does the moving car "observe"? (That's the frequency it will reflect.)
    2) What frequency does the police car observe?
  4. Apr 14, 2017 #3


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    Science Advisor
    Homework Helper
    Gold Member

    Where fL is what, exactly? Is that the frequency that the police will hear?

    Edit: pipped by The Doc.
  5. Apr 15, 2017 #4
    Thanks! I got it right now.
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