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JEE Main20205 Sep 2020Evening ShiftPhysicsWaves and SoundActual

A driver in a car, approaching a vertical wall notices that the frequency of his car horn has changed from 440 Hz to 480 Hz , when it gets reflected from the wall. If the speed of sound in air is 345 m s - 1 , then the speed of the car is:

Options

  1. A54   k m / h r
  2. B36   k m / h r
  3. C18   k m / h r
  4. D24   k m / h r

Correct answer

A. 54   k m / h r

Step-by-step solution

Given, Frequency of sound after reflection from the wall is f ' ' = 480   Hz Frequency of sound emitted by the horn of car is f = 440   Hz Speed of sound is v = 345   m   s - 1 The diver in the car (as a source) approaching the vertical wall which is the observer in this case, The frequency of sound received by the stationary wall is given by f r ' = v v - v c f     . . . ( 1 ) As the frequency f ' is received by wall will act as a source now, and driver in the car is an observer, the

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