Question

To estimate the height of a bridge over a 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 the bridge from the water level. Take a=g=9.8 m s - 2

Answer 1

Using displacement-velocity relationship
u=0 m/s
s= ut+1÷2 at×t
s=1÷2 ×at×t [as u= 0m/s]
s=1÷2 ×9.8×4
s=2×9.8=19.6 m
So,Height of bridge from the river is 19.6m

Answer 2

To get the height we calculate the vertical height
I.e.=> s= ut +1/2at^2
=> 4.9×4
=>19.6metres