A boy is standing on top of a tower of height 85 m and throws a ball in the vertically upward direction with a certain speed. If 5.25 s later he hears the ball hitting the ground, then the speed with which the boy threw the ball is (take, g = 10 m / s2 and speed of sound in air = 340 m/s)

Question

A boy is standing on top of a tower of height 85 m and throws a ball in the vertically upward direction with a certain speed. If 5.25 s later he hears the ball hitting the ground, then the speed with which the boy threw the ball is (take, g = 10 m / s2 and speed of sound in air = 340 m/s) (A) 6 m/s (B) 8 m/s (C) 10 m/s (D) 12 m/s

Tardigrade Admin · Accepted Answer

Given, sound of ball (hitting the ground) is heard

5.25 s

after the ball is thrown.

Sound of ball reaches top of tower in time,

t_{1} = \frac{D}{s} = \frac{85}{340} = 0.25 s

So, ball reaches ground in time

t_{2} = 5.25 - 0.25 = 5 s

If

u

= initial speed of ball at

t = 0

, then for the ball,

s = - 85 m, a = - g = - 10 m s^{- 2}, t = 5 s

Using,

s = u t + \frac{1}{2} a t^{2}

- 85 = u (5) + \frac{1}{2} (- 10) (5)^{2}

\Rightarrow 5 u = 125 - 85 \Rightarrow u = 8 m s^{- 1}

$6 m / s$

$8 m / s$

$10 m / s$

$12 m / s$

Q. A boy is standing on top of a tower of height 85m and throws a ball in the vertically upward direction with a certain speed. If 5.25s later he hears the ball hitting the ground, then the speed with which the boy threw the ball is (take, g=10m/s2 and speed of sound in air =340m/s)

6m/s

8m/s

10m/s

12m/s

$6 m / s$

$8 m / s$

$10 m / s$

$12 m / s$