First slide

Doppler's effect

Question

The driver of a car travelling with speed 30 metres per second towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is 330 metres per second, the frequency of the reflected sound as heard by the driver is

Difficult

A

720 Hz
B

555.5 Hz
C

550 Hz
D

500 Hz

Solution

From the figure, it is clear that
Frequency of reflected sound heard by the driver.
$n' = n [\frac{v - (- v_{o})}{v - v_{s}}] = n [\frac{v + v_{o}}{v - v_{s}}] = n [\frac{v + v_{car}}{v - v_{car}}]$
$= 600 [\frac{330 + 30}{330 - 30}] = 720 Hz .$

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