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Homework Help: Frequency of the sound heard by the passanger in the car

  1. Aug 11, 2014 #1
    1. The problem statement, all variables and given/known data
    The police car with its siren emits frequency of 828hz approaching a buliding at 20m/s. . What's the frequency reflected from the buliding ?
    the ans is ((340)/(340-20))x 828 = 880hz.
    What's the frequncy of the reflected sound incident on the approaching police car?
    The ans is ((340+20)/340) x 880 = 931hz.
    part iii) find the frequency of beats detected by the driver of the police car?
    the ans is 931-880= 51hz

    for part iii) why not the frequncy of beats =frequency of the reflected sound incident on the approaching police car(931hz) - frequency of the initial sound source (828hz) = 103hz ?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 11, 2014 #2


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    Gold Member

    I believe your answer is correct and the book answer wrong.
  4. Aug 11, 2014 #3


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    Staff: Mentor

    I agree with haruspex. The book's answer looks odd.
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