Question

A boy standing between two cliffs claps and hear two echoes, the first after 2 seconds and the second after 3 seconds. The velocity of sound is 360 m/sec. The distance in meter between the two cliffs is

• A. 900
• B. 360
• C. 1400
• D. 1200

Answer 1

Answer: (A)

900

Answer 2

Here boy is standing at point O between cliff (i) and (ii) $d_1$ and $d_2$ be the distances of boy to cliff (i) and (ii) respectively and let $v_s$ be the velocity of sound.
$\therefore \:\: d_1 = \frac{v_s \times t_1}{2}$ and $d_2 = \frac{v_s \times t_2}{2}$

Total distance $d = d_1 + d_2$
$= \frac{v_S \times t_1}{2} + \frac{v_s \times t_2}{2}$
$= \frac{v_s ( t_1 +t_2)}{2} = \frac{360}{2} \times (2 + 3) = 900 \, m$