The Doppler Effect - Example 2

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Stationary source, moving observer

A pupil is running towards school at 5m/s as they are late. The bell sounds at a frequency of 350 Hz. What frequency does the pupil hear the bell at? Take the speed of sound in air to be 340m/s.

Solution

Use the formula f'=\(\frac{v±vo}{v±vs}\)f

Because the source is stationary, vs =0

So now our formula looks like f'=\(\frac{v±vo}{v}\)f

The apparent frequency will sound louder as the pupil is approaching the source

f'=\(\frac{v±vo}{v}\)f = f'=\(\frac{340+5}{340}\)350 = 355 Hz

Example 2