First slide

Doppler

Question

A police car is chasing a motorcyclist at a velocity of 20m/s. velocity of motor cyclist is 15m/s. Wind is blowing from the police car towards the motor cyclist with a velocity of 30m/s velocity of sound in still air is 330m/s. If the motor cyclist blows horn of frequency 400Hz, The apparent frequency of sound received by the police car is

Moderate

Solution

Apparent frequency $f = \frac{f_{0} (1 + \frac{v_{m}}{v_{s} - v_{w}})}{(1 + \frac{v_{p}}{v_{s} - v_{w}})}$
$∴ f = \frac{400 (1 + \frac{15}{330 - 30})}{(1 + \frac{20}{330 - 30})} Hz = \frac{400 \times 315}{320} Hz = 393 .75 Hz$

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