Question

A whistle revolves in a circle with angular speed $\omega= 20 \,rad/sec$ 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 :-$\left( V _{\text {sound }}=340 m / s \right)$

• A. 385 Hz
• B. 374 Hz
• C. 394 Hz
• D. 333 Hz

Answer 1

Answer: (B)

374 Hz

Answer 2

$\omega=20\, rad / s,\, v =\omega r =20 \times 0.50=10\, m / s$
Minimum frequency $=385 \times\left(\frac{340}{340+10}\right)$
$=385 \times \frac{340}{350}=374\, Hz$