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Doppler Effect Question

  1. Apr 14, 2009 #1
    1. The problem statement, all variables and given/known data

    An object producing a sound with frequency 500 Hz is traveling at 40 m/s toward an observer, and the observer is traveling at 30 m/s away from the source. Another source producing a sound with the same frequency is traveling at 10 m/s toward a stationary observer. Determine the frequencies perceived by each of the observers, assuming that the speed of sound in air is 345 m/s. Are the two frequencies the same? If not, what causes the difference between the perceived frequencies?

    2. Relevant equations

    f' = f*(v+vo)/(v-vs)

    v = speed of sound = 345 m/s
    vo = speed of observer toward source
    vs = speed of source toward observer

    3. The attempt at a solution

    I've figured out that the frequency perceived by the first observer (traveling at 30 m/s) is 516 Hz, while the frequency perceived by the second observer (stationary) is 514.9 Hz.

    I understand mathematically why this happens. What I can't figure out is what physically occurs that would make the frequencies perceived by the two observers differ when the relative velocities between the observer and the source are the same in both cases.

    Thanks in advance for any help you can give me.
  2. jcsd
  3. Apr 15, 2009 #2


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    Homework Helper

    Frequency of the source always remains constant. Due to the motion of the source apparent wave length changes. If the observer is stationary, the same apparent wavelength is received by him. Mathematically f'/v = f/(v - vs). If the observer also in motion,the apparent wave length further changes. Mathematically f'/(v-vo) = f/(v - vs).
    In all these expression, the relative velocity between sound and source or sound and observer is considered, not the source and observer..
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