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Doppler effect on a police car?

  1. May 23, 2010 #1
    A police car's siren emits a wave with a frequency of 60 Hz. The speed of sound is 300 m/s and the car is moving 50 m/s. "what is the wavelength of the wave behind the car"

    So, the answer seems pretty easy. The observer is a stationary point behind the car, and the source is moving away from the observer, which means frequency should decrease:

    f = 60 (300+0)/(300+50) = 51.4 Hz
    wavelength = 300/51.4 = 5.8

    Is this logic correct?

    However, the answer in the book states that the final frequency is 70 Hz, by this equation:
    f = 60 (300+50)/(300+0) = 70 Hz

    Which one is correct?
  2. jcsd
  3. May 23, 2010 #2
    An increase in the frequency would indeed make no sense at all. :)
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