Two automobiles are equipped with the same single-frequency horn. When one is at rest and the other is moving toward the first at 12 m/s, the driver at rest hears a beat frequency of 5.5Hz. What is the frequency the horns emit?

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Answer 1 · 2017-12-22T18:03:55

The frequency is #=5.31Hz#

This is an exaple of the Doppler's effect

The source velocity is #v_s=12ms^-1#

The apparent frequency is #f'=5.5Hz#

The speed of sound in air is #v=340ms^-1#

The true frequency is

#f=(v-v_s)/v*f'#

#=(340-12)/(340)*5.5#

#=5.31Hz#