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Homework Help: The Doppler Shift - is this right?

  1. Mar 3, 2008 #1
    1. The problem statement, all variables and given/known data
    The Doppler shift was first tested in 1845 by the French scientist B. Ballot. He had a trumpet player sound an A 440 Hz while riding on a flatcar pulled by a locomotive. At the same time, a stationary trumpeter played the same note. Ballot heard 3.0 beats per second. How fast was the train moving toward him?

    2. Relevant equations
    f' = f/(1-Vs/V)

    3. The attempt at a solution

    Since it said that Ballot heard 3 bps, I added 3 to 440

    443 = 440/(1-Vs/V)
    >> 1-Vs/V = 440/443
    >> Vs = (440/443 - 1)-V
    >> Vs = (440/443 - 1)-343
    >> Vs = 2.32 m/s

    This doesn't seem right to me. I wasn't really sure if I should have added 3 to 440, but I didn't know how else to find the answer.
  2. jcsd
  3. Mar 4, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    It seems ok. The beat frequency is of course given by [itex] f_b = |f_2 - f_1|[/itex].
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