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A person carrying a whistle emitting continuously a note of $272 H z$ is running towards a reflecting surface with a speed of $18 k m / h o u r .$ speed of sound in air is $45 m / s .$ The number of beats heard by him is $:$

A

4

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B

6

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C

8

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D

3

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Solution

The correct option is C 8
According the concept of sound image $:$
$n^{'} = \frac{v + v_{p e r s o n}}{v - v_{p e r s o n}} \times 272$
$= \frac{345 + 5}{345 - 5} \times 272 = 280 H z$
$Δ n = N u m b e r o f b e a t s = 280 - 272 = 8$
Hence,
option $(C)$ is correct answer.

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