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NTA Abhyas JEE Main2020PhysicsWaves and SoundPractice

A train moving at a speed of 20 m s - 1 towards a stationary object emits a sound of frequency 930 Hz . Some part of the wave gets reflected from the object, back to the train as an echo. The frequency of this echo, as detected by the driver of the train is [speed of sound in air is 330 m s - 1 ]

Options

  1. A1400   Hz
  2. B1750   Hz
  3. C1050   Hz
  4. D1550   Hz

Correct answer

C. 1050   Hz

Step-by-step solution

The speed of the train, v T = 20 m s -1 The speed of sound in air, v = 330 m s -1 The frequency of the echo detected by the driver of the train is v ' = v v + v T v - v T v ' = 930 330 + 20 330 - 20 = 930 × 350 310   Hz ⇒ v ' = 1050   Hz

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