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Homework Help: Doppler effect

  1. May 28, 2010 #1
    1. The problem statement, all variables and given/known data

    A car is travelling at 30m/s in still air. the frequency of the siren on the car is 600hz. The velocity of sound in still air is 330 m/s. what is the wavelength of the sound waves

    a)directly in front of the car
    b)directly behind the car

    What is the frequency of the sound heard by a stationary observer

    c)directly in front of the car
    d)directly behind the car

    2. Relevant equations

    FD=CD/Cs *Fs

    3. The attempt at a solution

    Im not really sure where to start if someone could point me in the right direction.

  2. jcsd
  3. May 28, 2010 #2
    OK, let's kick this off. Can you first get the frequency of the doppler shifted sound wave? I think your equation above (though you haven't defined what each variable represents) should tell you that.
  4. May 28, 2010 #3
    Also, write out how to get your C's.
  5. May 28, 2010 #4
    well i can find the wavelength of the car but i think its only when its stationary im not sure

    If Cs of the source is 30 m/s an the frequency is 600 hz
    then the wavelength is

    0.05 m

    but im not so sure where to go from there.. if i have Cs and Fs how do i find what the wavelength in front and behind is
  6. May 28, 2010 #5
    actually i think i have the answer.. i used an equation that was stated in my text book..

    Wavelength behind = (vm+us)/Fs

    where Vm= velocity of sound in medium
    Us= velocity of source

    and it gets an answer of 0.6 m


    wavelength infront = (vm-us)/Fs

    given an answer of 0.5 m

    and from here i can calculate the frequency of each wavelength from V= F*(Lambda)
  7. May 28, 2010 #6
    Awesome! Have a good one.
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