First slide

Doppler

Question

A whistle revolves in a circle with angular speed $ω = 20$ rad /sec c 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 center $\cdot (v_{sound} = 340 m / s)$ .

Moderate

A

385 Hz
B

374 Hz
C

394 Hz
D

333Hz

Solution

$Linear velocity V = rω = (50 \times 10^{- 3}) \times 20 = 10 m / s When the source is moving away from the observer n^{'} = n (\frac{V_{sound}}{V + V_{sound}}) = 385 \times (\frac{340}{10 + 340}) = \frac{385 \times 34}{35} = 374 Hz$

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