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 5.25 secs later he hears the ball hitting the ground, then the speed with which the boy threw the ball is ( take g=910m/s^{2}, speed of sound in air=340m/s)

- 6m/s
- 8m/s
- 10m/s
- 12m/s

Solution:

Time taken by sound= 85/340=0.25secs

Time taken by the ball= 5.25-0.25=5 sec

Now,

we write the equation $$ s=ut+1/2ft^2 $$

Here s=-85 and time t =5 secs

Hence,

$$ -85=5u-1/2*10*25$$

$$\Rightarrow u=8m/s$$