Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Doppler Effect Ahhhh!

  1. Dec 2, 2007 #1
    Doppler Effect....Ahhhh!

    1. The problem statement, all variables and given/known data
    A student in a parked car honks the horn, which has a `proper' frequency of 310.00 Hz. An observer in an approaching vehicle measures the frequency of the sound to be 342 Hz. Calculate the speed of the approaching vehicle. Use 340 m/s as the speed of sound in air.

    2. Relevant equations
    Fo / V + Vo = Fs / V + Vs
    Fs = 310 Hz
    Vs = 0 m/s
    Fo = 342 Hz
    Vo = ?
    V = 340 m/s

    3. The attempt at a solution
    Vo = (Fs / V + Vs) * (V / Fo)
    I know this is where I am messing up because I am not getting the right answer. Can someone show me where I am going wrong.
  2. jcsd
  3. Dec 2, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Be careful in grouping terms.

    One should have a form of the equation as:

    [tex]f_o\,=\,f_s(\frac{v\pm{v_o}}{v\pm{v_s}})[/tex] where subscripts o and s refer to observer and source, respectively, and v is the speed of sound.

    Please refer to

    http://www.kettering.edu/~drussell/Demos/doppler/doppler.html [Broken]

    Here is a nice tutorial on sound waves

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/soundtoc.html [Broken]
    Last edited by a moderator: May 3, 2017
  4. Dec 2, 2007 #3
    What if I am looking for the speed. And for this one how I do I determine what is the observer F and V and the source F and V?

    A person standing close to a railroad crossing hears the whistle of an approaching train. He notes that the pitch of the whistle drops as the train passes by and moves away from the crossing. The frequency of the approaching whistle is 517 Hz, and drops to 494 Hz after the train is well past the crossing. What is the speed of the train? Use 340 m/s for the speed of sound in air.
  5. Dec 2, 2007 #4
    Does anyone know how to do this?
  6. Dec 2, 2007 #5
    so if we want to find the Velocity we can do...
    (Fs +/- Fo) / (Fs +/- Fo) * V = Vs
    (Fo +/- Fs) / (Fo +/- Fs) * V = Vo

    how do I know if its +/-? I know it depends on if the source is approaching or receding but what sign corresponds to which?
  7. Dec 2, 2007 #6

    Doc Al

    User Avatar

    Staff: Mentor

    In the equation provided by Astronuc, V is the speed of sound, Vo is the speed of the observer, and Vs is the speed of the source. Fs is the frequency of the source and Fo is the observed frequency.

    If source and observer approach each other, Fo > Fs; if they separate, Fo < Fs.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook