An observer stands at a distance of $850 m$ from the cliff and fires a gun. After what time gap will he hear the echo, if sound travels at a speed of $350 m s^{- 1}$ in air?

$6.89 s$

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$4.00 s$

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$2.93 s$

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$4.86 s$

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Solution

The correct option is D
$4.86 s$

Given:
Speed of sound, $v = 350 m s^{- 1}$
Distance between the observer and the cliff, $d = 850 m$

Distance travelled $s = 2 d$
because sound travels from the observer to the cliff and back to the observer.

$Speed = \frac{Distance}{Time}$
$v = \frac{2 d}{t}$
$t = \frac{2 d}{v}$
$t = \frac{2 \times 850}{350} = 4.86 s$

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The time interval after which will he hears the echo is:

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{330 m s}^{- 1}

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