Question

A boy stands $85m$ in front of a high wall and then blows a whistle. Calculate the time interval when he hears an echo. (speed of sound $=340m{s}^{-1})$

• A. $0.1s$
• B. $0.2s$
• C. $0.5s$
• D. $1s$

Answer 1

The correct option is C $0.5s$

Here we use the concept of Distance formula i.e., $d=vt$ , where $d$ is the distance travelled, $v$ is the speed and $t$ is the time taken.

Now, According to question;

$v=340m/s$

and Total distance travelled by the sound is, $85\ast 2\cong 170m$ , because sound will first move toward the wall then it reflect from wall and move towards the boy/observer and he hears an echo. So Total Distance covered by the sound is, $85\ast 2=170m$

$⟹$ $d=170m$

Therefor, We have to find Time Taken$\left(t\right)$ and i.e., $t=d/v=170/340=1/2=0.5s$

So, Time Taken$\left(t\right)$ is, $0.5second\cong 0.5s$

Hence, option C is correct.