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Question

A whistle revolves in a circle with angular velocity ω=20 rad/s using a string of length 50 cm. If the actual frequency of sound from the whistle is 385 Hz, then the minimum frequency heard by the observer far away from the centre is (velocity of sound v=340 m/s)


Your Answer
A
385 Hz
Correct Answer
B
374 Hz
Your Answer
C
394 Hz
Your Answer
D
333 Hz

Solution

The correct option is B 374 Hz
The diagram shows the velocities of the source at different points along its circular path.


There will be no change in frequency heard by the observer at points B and D since the velocity of the source line joining the source and observer.

vsource=ωr=200.5=10m/s

At point A, there will be an increase in frequency

Minimum frequency will be heard at point C since source is moving away from observer.

fC=f0(vwavevwave+vsource)

=385(340340+10)=374 Hz
 

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