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Question

A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the apparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km/h? Speed of sound in air = 340 m/s.
  1. 2.5 kHz 

  2. 20 kHz 

  3. 2.06 kHz

  4. None of these 


Solution

The correct option is C

2.06 kHz


Let u = velocity of sound;

u0= velocity of the observer;                                                   us = velocity of the sources

f=v+u0vusf0
Where,
V = speed of sound in the medium,
u0 = speed of the observer with respect to the medium, considered positive when it moves towards the source and negative when it moves away from the source
and us =  speed of the source with respect to the medium, considered positive when it moves towards the observer and negative when it moves away from the observer.
It should be carefully noted that the speeds us and u0 are to be written with respect to the medium carrying the sound. If the medium itself is moving with respect to the given frame of reference, appropriate calculations must be made to obtain the speeds of the source and the observer with respect to the medium.

v = 340 m/s, us = 0 and uo = 10 m/s
f`= 2.06 kHz

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