A train blows a whistle of 480 Hz as it runs at 30 m/s…

Contributed by StemPy Lab

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

A train blows a whistle of $480\ \text{Hz}$ as it runs at $30\ \text{m/s}$ . A car on a road beside the track drives towards the train at $20\ \text{m/s}$ . Take the road and the track as one straight line. What does the driver of the car hear (a) before the two pass each other and (b) after they have passed?

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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