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Sound interference and whistle

  1. Jan 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A car honks a 2kHz tone and is 40m from a long high wall. A man is running parallel to the wall, 10m away from it, at 5 m/s, and is presently 30m away from where the whistle is closest to the wall. How many beats/second will be heard by the runner as he will hear the direct whistle signal and the one reflected from the wall.


    2. Relevant equations
    3. The attempt at a solution

    I have drawn a diagram and also knw that this problem has to do with the Doppler effect.
    But i am not sure where to start.
     

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  2. jcsd
  3. Jan 1, 2013 #2

    haruspex

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    You can treat the echo as coming from another source, being the image of the car reflected in the wall. How fast is the man moving away from the car? From the image car?
     
  4. Jan 1, 2013 #3
    okay so i know that the man is moving 5m/s and the frequency at the source will be
    2kHz but how would i found the seed of the wave...will it just be speed of sound?
     
  5. Jan 1, 2013 #4

    haruspex

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    He's moving at 5m/s, but how fast is he moving away from the car?
     
  6. Jan 1, 2013 #5
    aright the man is 42.42m away from the car and 31.62m away from the reflection...but would he not be moving away at 5m/s from both these points aswell?
    if not how would u figure that out?
     
  7. Jan 2, 2013 #6

    haruspex

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    Velocity is a vector. What is the component of the man's velocity along the line from car to man?
    What do you mean by "the man is 31m from the reflection? It's the distance and speed from the car's 'image' reflected in the wall that matters.
     
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