Question

To estimate the height of bridge over a river, stone is dropped freely on the river from the bridge. The stone takes 2s to touch

Answer 1

S= ut+1/2 at2
= 0x2+1/2x9.8x2x2
=19.6 mtrs

pls try to solve by applying Newtons laws 
here  u= Initial velocity =0 m/s
          free falling acceleration =g=9.8 m/sec2
time taken to touch water =2 sec

Answer 2

$\huge\star\sf\underline\pink{Question}$

To estimate the height of a bridge over river, a stone is dropped freely in the river from the Bridge. The stone takes 2 seconds to touch the water surface in the river. calculate the height of a brief from the water level.

$\huge\star\sf\underline\pink{Answer:-}$

$\sf{We\:Have,} \\ \\ \sf{ Initial\: Velocity\:of\:stone(u) = 0} \\ \\ \sf{Time\:Taken(t) = 2s} \\ \\ \sf{Acceleration\:due\:to\:the\: gravity (g) = 9.8\: m/s^2 }$

$\sf\underline\red{Formula:-} \\ \\ \: \: \: \: \bullet\sf{Height\:of\:the\: Bridge (h) =?}$

We know,

$\blue{\boxed{\sf{ Height (h) = ut + \dfrac{1}{2} gt^2 }}}$

Now,

After putting Values in the Formula, we get:

${\sf{h = 0 \times 2 + \dfrac{1}{2} \times 9.8 \times (2)^2 }} \\ \\ \\ \longrightarrow{\sf{ h = \dfrac{1}{2} \times 9.8 \times 4}} \\ \\ \\ \longrightarrow\sf{ h = 19.6\:m}$

Thus,

The height of Bridge is 19.6 Metres.