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Homework Help: Doppler Effect

  1. May 22, 2010 #1
    1. The problem statement, all variables and given/known data

    (a) 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,
    (ii) when the wind blows from the siren to the person?

    2. Relevant equations


    3. The attempt at a solution

    i figured out the relative velocity of the sound (for i 343-15) and then subbed into the formula which became 1+15/328=1.04573HZ. I'm pretty sure this isnt right because the frequency should get higher but mine inst even close.... also i didnt use the given frequency at all????????
  2. jcsd
  3. May 23, 2010 #2


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    Homework Helper

    Check the formula.
  4. May 23, 2010 #3
    That formula is the one in my text book for the observer moving and the source stationary
  5. May 23, 2010 #4
    hey, i really need some help with this if you know a different equation i should use?
  6. May 23, 2010 #5


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    Homework Helper

    The correct formula is

    [tex]f' = [1 + \frac{v_o}{v}]f[/tex]

    Velocity of sound will be depending on the direction of the wind.
  7. May 23, 2010 #6
    ok thanks i dont know how i missed the second f...thanks
  8. May 27, 2010 #7
    I get (i) 941Hz and (ii) 938Hz

    Does this sound right?
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