Question

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 $350m{s}^{-1}$ in air?

• A. 4.86 s
• B. 6.89 s
• C. 4.00 s
• D. 2.93 s

Answer 1

The correct option is A 4.86 s

Given, speed of sound, $v=350m{s}^{-1}$ and distance between the observer and the cliff, $d=850m$.

This is a case of an echo, so distance travelled = 2d (because sound travels from the observer to the cliff and back to the observer)
We know, $speed=\frac{distance}{time}$
$v=\frac{2d}{t}$
$\Rightarrow t=\frac{2d}{v}$

$=\frac{(2\times 850)}{350}=4.86s$.