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Homework Help: Deriving A Formula

  1. Dec 3, 2009 #1
    1. The problem statement, all variables and given/known data
    This is my first post, so it might not be that great... Anyways, I need help deriving the formula. This formula is pertaining to the doppler effect, and more specifically, to find the frequency of a sound made from an object moving toward/away from you.

    f2= f1(v/v-vs) or... f1(v/v+vs)

    Where f2 is the second frequency, f1 is the first frequency, v is velocity, and vs is the velocity of the source.

    2. Relevant equations

    v=(lambda)(frequency) (I don't know how to make any symbols =/)
    beat frequency= absf1-f2endabs

    3. The attempt at a solution

    What I did was try to make every variable a frequency, and I had no idea what to substitute in for vs...
  2. jcsd
  3. Dec 5, 2009 #2


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    Homework Helper

    I don't think you need the beat frequency formula.

    Think about the problem like this. At a certain moment in time, a siren emits a crest (or trough; take your pick), and the energy spreads out in all directions at the same speed. After one period passes, it emits another crest, which again spreads out in all directions. If the siren's moving, this crest will be closer to the initial crest in the direction of motion and farther from it in the opposite direction. If you can find the distance between the two crests, that's your wavelength.
  4. Dec 6, 2009 #3
    I think I might've worded my question wrong; I'm trying to prove the formula itself, I don't need to find the wavelength; unless that is what I am suppose to do?
  5. Dec 7, 2009 #4
    I don't think lambda was suppose to use in the original equation, was it? I forgot to add that part in my original question.
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