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Homework Help: Doppler Effect Question regarding wavelength changing

  1. Apr 21, 2013 #1
    1. The problem statement, all variables and given/known data

    A listener L hears the siren from a moving police car ,S, which is moving away from the listener with a speed of v = 20.0 m/s. The listener is moving at 10.0 m/s towards the car. There is a wind blowing with a speed of v(wind)= 10.0 m/s from S to L. The siren is emitting sound at a frequency (f) of 800 Hz. The velocity of sound in air is 343 m/s. Calculate the frequency (f') heard by the listener and the wavelength (λ) of that wave.

    2. Relevant equations

    f' = f(v +/- vD) / (v -/+ vS)
    v = fλ

    3. The attempt at a solution

    Finding the new frequency is easy:

    f' = (800hz)(343+10+ 10)/(343+10+20)
    f' = 779 Hz.

    But now for finding the wavelength I'm confused.
    I know it's λ = v/f'

    but is it:
    v = 343m/s (the speed of sound in still air)
    v= 343+10 m/s (the speed of sound + the speed of the wind)
    v= 343+10+20 m/s (the speed of sound + the speed of the wind + the speed of the source)
    or something else?

    Any help would be much appreciated.
    Last edited: Apr 21, 2013
  2. jcsd
  3. Apr 21, 2013 #2

    Simon Bridge

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    Science Advisor
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    The wavelength heard by the listener is the frequency of the sound divided by the speed the wave is passing the listener.
  4. Apr 21, 2013 #3
    My apologies, I put the wrong question (I had v = f/λ when it should be v= fλ); I fixed this up now.

    But by your statement would be the last option I suggested? λ = (speed of sound + speed of wind + speed of source)/(new frequency)?
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