A whistle revolves in a circle with angular speed = 20 rad/s using a string of length 50 cm. If the frequency of sound from the whistle is 385 Hz, then what is the minimum frequency heard by an observer which is far away from the centre (velocity of sound = 340 m/s)
- 385 Hz
- 374 Hz
- 394 Hz
- 333 Hz
Answer
(2) 374 Hz
Solution
Speed of the whistle: vs = r = 0.5 20 = 10 m/s
∵ The observer is far away, the minimum frequency is heard when the whistle moves directly away from the observer.
f = f0 vv + vs
= 385 340340 + 10
= 385 340350
= 374 Hz
Answer: (2)