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Homework Help: Doppler Effect

  1. Feb 7, 2010 #1
    1. The problem statement, all variables and given/known data
    Assume that the speed of sound in the air is V=343m/s. Use the generalized form of the Doppler equation to solve the following problem.
    You are standing 100m away from a long straight road while a fire engine passes by along the road. The fire engine is equipped with a siren which emits a steady frequency of 440Hz. If the fire engine is travelling at 80km/h along the road, what frequency do you preceive for the siren
    (a) 100m (measured along the road) before the fire engine passes
    (b) 50m (measured along the road) after the fire engine passes

    2. Relevant equations

    generalized form of the doppler equation: f ' = [(V + Vocos(θo))/(V-Vscos(θs))]f

    3. The attempt at a solution

    (b) if the source and the observer are moving away from each other, we have: θs - θo = 180 and since cos180 = -1, we get the equation f ' = [ V/(V+Vs)]f with negative values for both Vo and Vs.
  2. jcsd
  3. Feb 8, 2010 #2
    I think that in your question the observer is not moving .. your equation is correct f` = (v/(v+vs))f , was that your question or you have other questions?
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