Question
A horizontal bridge is built across a river. A student standing on the bridge throws a small ball vertically upwards with a velocity $4\,m s ^{-1}$. The ball strikes the water surface after $4\,s$. The height of bridge above water surface is $......\,m$ $\left(\right.$ Take $\left.g=10\,m s ^{-2}\right)$

Answer

$S = ut +\frac{1}{2} a t^2$

$-H =4 \times 4-\frac{1}{2} \times 10 \times 4^2$

$-H =16-80$

$-H =-64$

$H =64\,m$

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