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JEE Main201910 Jan 2019Morning ShiftPhysicsWaves and SoundActual

A train moves towards a stationary observer with speed 34 m / s . The train sounds a whistle and its frequency registered by the observer is f 1 . If the speed of the train is reduced to 17 m / s , the frequency registered is f 2 . If speed of sound is 340 m / s , then the ratio f 1 / f 2 is:

Options

  1. A21 / 20
  2. B20 / 19
  3. C19 / 18
  4. D18 / 17

Correct answer

C. 19 / 18

Step-by-step solution

For stationary observers, f = v v - u × o r i g i n a l   f r e q u e n c y f 1 = 340 340 - 34 f f 2 = 340 340 - 17 f f 1 f 2 = 340 - 17 340 - 34 = 19 18

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