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Frequency of the echo heard by a driver moving towards a wall or hill

This type of question, asked 3 times: 2021 Paper 11, 28 Jul 2022, 6 Apr 2023. All 3 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    2021 Paper 11

    8
    A car is moving towards a stationary wall sounding a horn of frequency 400 Hz400\text{ }Hz. The reflected frequency heard by the driver of the car is 500 Hz500\text{ }Hz. Find the speed of the car. [V=V= speed of sound]

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  2. Question 2

    28 Jul 2022

    27
    The frequency of echo will be ____\_\_\_\_ HzHz if the train blowing a whistle of frequency 320 Hz320\text{ }Hz is moving with a velocity of 36 km/h36\text{ }km/h towards a hill from which an echo is heard by the train driver. Velocity of sound in air is 330 m/s330\text{ }m/s.

  3. Question 3

    6 Apr 2023

    25
    A person driving car at a constant speed of 15 m/s15\text{ }m/s is approaching a vertical wall. The person notices a change of 40 Hz40\text{ }Hz in the frequency of his car's horn upon reflection from the wall. The frequency of horn is___ Hz. (Given: Speed of sound: 330 m/s330\text{ }m/s )