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Homework Help: High School Physics: Doppler Effect

  1. Jul 5, 2010 #1
    1. The problem statement, all variables and given/known data

    An automobile is approaching you at a speed of 60.0 km/h and sounding its horn. The fundamental frequency of the horn sounds to you like 388 Hz. If the real fundamental frequency of the horn is 369 Hz, what is the speed of sound?

    2. Relevant equations

    Doppler Frequency = (speed of sound / (speed of sound - speed of source)) source frequency

    fd = (vw/ vw - vs) fs

    3. The attempt at a solution

    What I'm having trouble with is rearranging the formula to solve for the speed of sound. Because there is two 'speed of sound' variables, you either end up dividing one by the other, and eliminating all variables; or subtracting one from the other, and (once again) eliminating all variables. As far as I can see, it's not possible?

    Could someone help me out with rearranging this formula properly?

    In case anyone's wondering, it's Grade 11 Physics.
  2. jcsd
  3. Jul 6, 2010 #2
    You have the formula written down wrong.

    You wrote:

    fd = (vw/ vw - vs) fs

    It's supposed to be:

    fd = (vw / (vw - vs)) fs

    Good luck!
  4. Jul 6, 2010 #3

    Filip Larsen

    User Avatar
    Gold Member

    Try multiply both sides of the equation (when its written up correctly as Tusike noted) with vw-vs. The resulting equation should now be easier to solve for vw.
  5. Jul 6, 2010 #4
    Thanks alot! I got the correct answer, thanks to both of those pieces of advice.
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