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An ambulance with a siren of 720 Hz drives at 25 m/s along a straight road past a man standing on…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

An ambulance with a siren of $720\ \text{Hz}$ drives at $25\ \text{m/s}$ along a straight road past a man standing on the pavement. Find the frequency he hears (a) as it comes towards him and (b) after it has passed. (c) By how much does the note drop?

Need a reminder? The key ideaShow

With the source and listener moving along the line between them, speeds measured in the air:

\displaystyle f' = f\,\frac{v \pm v_o}{v \mp v_s}

Upper signs when that body moves towards the other. An echo from a reflector coming towards you at $u$ : $f\,\dfrac{v + u}{v - u}$ .

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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