# Homework Help: Sound interference and whistle

1. Jan 1, 2013

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. Jan 1, 2013

### haruspex

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?

3. Jan 1, 2013

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?

4. Jan 1, 2013

### haruspex

He's moving at 5m/s, but how fast is he moving away from the car?

5. Jan 1, 2013