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Doppler effect help please

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data
    A siren emits a frequency of 900 Hz. Wind is blowing at a steady speed of 15.0 ms-1. The speed of sound in calm air is 343 ms-1. What is the frequency heard by someone approaching at 15.0 ms¹,
    (i) when the wind blows from the person to the siren, and
    (ii) when the wind blows from the siren to the person?
    2. Relevant equations

    f^'=f/((1-v_s/v) ) source approaching

    3. The attempt at a solution
    for i) i figured that since the sound is no longer in calm air that the velocity v_s would be 343-15=328m/s, and v would be 15m/s.. so that comes out to -.1916Hz????????????? frequency cant be negative can it? i imagine ii) is the same method??? please help
  2. jcsd
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