Question

A man fired a bullet against a wall and hears an echo after 2s. He walks 80 m towards the wall and fired bullet, such that he hears echo after 1s. Find the distance from wall to the second fired place.

Answer 1

The distance of the wall is 2 ×80 = 120m because the time of hearing of echo is half when he moves 80m towards the wall it keans that he coverd half of the distance because the time of echo is half

Answer 2

$\sf\huge\bold{Answer:}$

From the given data we have,

$d _{walk}$=80m

$t _{1}$=2s

$t_{2}$=1s

$d _{2}$=?

v= velocity of sound

When he fires the bullet, the sound generated travels everywhere, including in the direction towards the wall. 

The sound wave travelling towards the wall, hits it, gets reflected from it, moves backwards and reaches his ear.

This is how he hears the echo.

This shows that the sound wave travels twice the distance between him and the wall. Hence, while calculating, we'll have to double the distances.

$⟶2d _{1} = v \times t _{1}...(i)$

$⟶2d _{2} = v \times t _{2}...(ii)$

$⟶d _{1} = d _{2} + d _{walk}...(iii)$

Step 1 : Diving equation ii by i

$⟶ \frac{d _{2}}{d _{1}} = \frac{t _{2}}{t _{1}} ...(iv)$

Step 2 : Substituting 3 in 4

$⟶\frac{d _{2}}{d _{2} +d _{walk} } = \frac{t _{2}}{t _{1}}$

$⟶ \frac{d _{2}}{d _{2} +80 } = \frac{1}{2}$

$∴2d _{2} = d _{2} + 80$

$∴d _{2} = 80m$

⇒ the distance from wall to the second fired place is 80 metres.