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Homework Help: Doppler Effect question

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data

    A car speeds towards the outside wall with its horn sounding a frequency of 160 HZ. The sound waves reflected off the wall cause the driver to hear 8.0 beats per second. There is no wind and the speed of sound is 343 m/s. Find the speed of the car.

    2. Relevant equations

    3. The attempt at a solution

    Well with the beats being 8, ive reckoned that the sound that the driver hears is either 152 or 168 HZ. But neither of those two frquencies lead me to the correct answer. Ive thought that the frequency the driver hears should be 168 HZ because the freqquency goes up in situations like that.

    What do you guys/girls think?
    Last edited: Dec 15, 2009
  2. jcsd
  3. Dec 16, 2009 #2
    You are correct about the beats meaning that he hears either 152 or 168 Hz reflecting back from the wall.
    Don't forget, though, that the Doppler effect has influenced this value twice.
    The speed of travel towards the wall has increased the frequency with which the sound waves strike the wall, and are reflected.
    But the car then meets these reflected waves while, itself, travelling towards the wall.
    Does this influence your result?
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