First slide

Doppler

Question

A train moves towards a stationary observer with a speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is $f_{1}$ . If the train's speed is reduced to 17 m/s the frequency registered is f₂. If the speed of sound is 340 m/s then the ratio f₁/ f₂is

Moderate

A

18/19
B

1 /2
C

2
D

19 /18

Solution

When the train moves towards the stationary observer, then apparent frequency /1, is given by
$f_{1} = \frac{vf}{v - v_{s}} = \frac{340 f}{340 - 34} = \frac{340 f}{306} further, f_{2} = \frac{Vf}{V - V_{S}} = \frac{340 f}{340 - 17} = \frac{340 f}{323} \frac{f_{1}}{f_{2}} = \frac{323}{306} = \frac{19}{18}$

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