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Question

A car moves at a speed of 20ms-1 sounding its horn at 480Hz. If the speed of sound is 340ms-1, the frequency of the reflected sound heard by the girl sitting in the car will be closest to


A

540Hz

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B

524Hz

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C

568Hz

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D

480Hz

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Solution

The correct option is A

540Hz


Step 1. Given data

Frequency of the horn, f=480Hz

Speed of the car, vs=20ms-

Speed of the sound,v=340ms-1

Step 2. Finding the Apparent frequency before reflection

By using the formula of Apparent frequency, f'

f'=vv-vs×f

f'=340340-20×480

f'=510Hz

Step 3. Finding the Apparent frequency after reflection

After reflection, the girl sitting in the car act as an observer

Apparent frequency after reflection, f''

f''=v+vov×f'

f''=340+20340×510

f''=540Hz

Therefore, the frequency of the reflected horn heard by girl is 540Hz

Hence, the correct option is A.


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