A traffic policeman sounds a whistle to atop a car-driver approaching towards him. The car-driver does not stop and Lakes the plea in court that because of the Doppler shift, the frequency of the whistle reaching him might have gone beyond the audible limit of 20 kHz and he did not hear it. Experiments showed that the whistle emits a sound with frequency close to 16 kHz. Assuming that the claim of the driver is true, how fast was he driving the car’ Take the speed of sound in air to be 330 m s -1 . Is this speed practical with today’s technology?

A traffic policeman sounds a whistle to atop a car-driver approaching towards him. The car-driver does not stop and Lakes the plea in court that because of the Doppler shift, the frequency of the whistle reaching him might have gone beyond the audible limit of 20 kHz and he did not hear it. Experiments showed that the whistle emits a sound with frequency close to 16 kHz. Assuming that the claim of the driver is true, how fast was he driving the car’ Take the speed of sound in air to be 330 m s^-1. Is this speed practical with today’s technology?

As he was moving towards the source, the Doppler shift can be found out by,

Here,

f₀= 16 KHz

v=330 ms^-1

f=20kHz

So he must travel at a speed of more than 82.5 ms^-1 or 297 kmh^-1

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