First slide

Doppler

Question

A train moving at a speed of 220 ${ms}^{- 1}$ towards a stationary object emits a sound of frequency 1000 Hz. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is
(speed of sound in air is 330 ${ms}^{- 1}$ )

Moderate

A

3500 Hz
B

4000 Hz
C

5000 Hz
D

3000 Hz

Solution

From Doppler's shift, we know for this case
$n^{'} = n (\frac{v + v_{s}}{v - v_{s}}) = 1000 (\frac{330 + 220}{330 - 220})$
= $1000 (\frac{550}{110}) = 5000 Hz$

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