Question

To estimate the height of the bridge over a river, stone is dropped freely in the river from the bridge. The stone takes 2.5 seconds to touch the water side facing the river calculate the height of the bridge from water level

Answer 1

u=0
t=2.5s
a=9.8 m/s²
v=u+at
v=0+9.8×2.5
v=24.5
now we will use the equation 2as=v²-u²
so s=v²-u²/2a
so s=(24.5)²-0/2×9.8
s=600.25/19.6
s=30.625m

Answer 2

time taken for free fall from a height h is given as 

$t = \sqrt{\frac{2h}{g}}$

from this , h = $\frac{1}{2}gt^2$.

Given that t =2.5 s and lets take g = 9.8 $ms^{-2}$.
Then, h = $\frac{1}{2}\times 9.8\times (2.5)^2 = 30.625 m$