Question

A man standing between two cliffs hears the first echo of a sound after 2 sec and the second echo 3 sec after the initial sound. If the speed of sound be $330m/sec$ the distance between the two cliffs should be

• A. 1650 m
• B. 990 m
• C. 825 m
• D. 656 m

Answer 1

The correct option is C 825 m

Let $d_1$ and $d_2$ be the distances of first and second cliff from man .

Echo after 2s will be heard when sound travels a distance of $2d_1$ , because an echo comes back to initial point after reflection .

therefore by , distance=speed$\times$time ,

or $2d_1=330\times 2=660$ ,

or $d_1=660/2=330m$ ,

similarly Echo after 3s will be heard when sound travels a distance of $2d_2$ ,

therefore

or $2d_2=330\times 3=990$ ,

or $d_2=990/2=495m$ ,

as the man is in between the cliffs , therefore distance between cliffs willbe ,

$d=d_1+d_2=330+495=825m$