First slide

Doppler

Question

The frequency of a train whistle is 360 Hz. As the train passes t}rough a station the frequency appears to be 400 Hz to a man who is standing on the station platform. If the speed of sound in air is 340 m/s, the speed of train is

Moderate

A

20m/s
B

34 m/s
C

36 m/s
D

39 m/s

Solution

$n^{'} = (\frac{V}{V - V_{s}}) n 400 = (\frac{340}{340 - v_{s}}) 360 o r 400 \times 340 - 400 V_{s} = 340 \times 360 o r 340 [400 - 360] = 400 v_{s} \frac{340 \times 40}{400} = v_{s} V_{s} = 34 m / s$

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