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Homework Help: Qn on classical doppler effect

  1. Oct 16, 2005 #1


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    I've got 2 qns
    Student holding a tuning fork vibrating at 440Hz walk away from a wall with a speed of 1.2m/s. What's the freq of the echo the student can hear from the wall? Speed of sound is 330m/s

    Can i just use Freq= [(330-1.2)/(330+1.2)] (440Hz) to solve
    2) A car travelling at 10m/s sound its horns, which has a freq of 500Hz and this is heard by another car travelling behind the 1st car in the same direction but at a speed of 20 m/s.The sound can also be heard by the 2nd car by the reflection from the bridge ahead. What frequency will the 2nd car hear? Take speed of sound be 340m/s.

    Is the frequency they are asking beat freq?
    How do i cal. the freq due to the doppler effect? Must i make use of relative velocity?
  2. jcsd
  3. Oct 16, 2005 #2

    Chi Meson

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    1) No they don't want the beat frequency, but in this case the walking person is both the source AND the observer (and both source and observer are moving)

    2) Same thing; think of the situation as though the observer is the reflection in a mirror attached to the bridge. Use observer direction as it appears in the reflection.
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