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A boy is walking away from a wall towards an observer at a speed of 1 metre/sec and blows a whistle whose frequency is 680 Hz. The number of beats heard by the observer per second is (Velocity of sound in air = 340 metres/sec

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Detailed Solution

Observer hears two frequencies

(i) n₁ which is coming from the source directly

(ii) n₂ which is coming from the reflection image of source

so, $n_{1} = 680 (\frac{340}{340 - 1})$ and $n_{2} = 680 (\frac{340}{340 + 1})$

$\Rightarrow n_{1} - n_{2} = 4 beats$

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