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 the second man will hear the echo from the cliff?
(Speed of sound = $300\text{m/s}$)

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

Answer 1

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

Given:
Speed of sound, $v=300\text{m/s}$
Since the speed of light is very high as compared to the speed of sound, so the flash of a gunshot will reach a second man within no time.
According to the question, let the distance traveled by the gunshot sound coming from the gun to the cliff and reflecting from the cliff to the second man be $d$.

From the above diagram, the value of $d$ is $150+150+300=600\text{m}$
We know that $v=\frac{d}{t}$ or $t=\frac{d}{v}$
So, the time after which the second man will hear the echo of the gunshot from the cliff after he sees the flash, $t=\frac{600}{300}$
$⟹t=2\text{s}$
So the second man first hears the sound of gunshot after $1\text{s}$ of the flash of gunshot and he hears the echo of the gunshot from the cliff $2\text{s}$ after the flash of gunshot appears. We are neglecting the time taken by the flash of gunshot to reach the person's eye as it is very less.