Question

A man fires a gun at a distance of $150\text{m}$ from a cliff. A second man who is standing $300\text{m}$ away from the first man hears the gunshot $1\text{s}$ after he sees the flash. When will the second man hear the echo from the cliff? (Take the speed of sound as $300\text{m/s}$.)

• A. $2\text{s}$ after seeing the flash
• B. $2\text{s}$ after hearing the gunshot
• C. $1\text{s}$ after seeing the flash
• D. $4\text{s}$ after hearing the gunshot

Answer 1

The correct option is A $2\text{s}$ after seeing the flash

Distance between first and second man, $S_1=300\text{m}$
Time taken for second man to hear the gunshot after the flash, $t_1=1\text{s}$
Velocity of sound, $V=\frac{S_1}{t_1}$
$V=\frac{300}{1}=300\text{m/s}$
Distance travelled by the sound to reach the second man after reflecting from the cliff,
$S_2=150+150+300=600\text{m}$
Time taken for the echo to reach the second man,
$t_2=\frac{S_2}{v}$
$t_2=\frac{600}{300}=2\text{s}$
The second man will hear the echo $2\text{s}$ after the flash and $1\text{s}$ after hearing the gunshot.