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  4. A ball is dropped from a bridge of 122.5 metre above a river. After the ball has been falling for two seconds, a second ball is thrown straight down after it. Initial velocity of second ball so that both hit the water at the same time is

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Q. A ball is dropped from a bridge of $122.5$ metre above a river. After the ball has been falling for two seconds, a second ball is thrown straight down after it. Initial velocity of second ball so that both hit the water at the same time is

Motion in a Straight Line

Solution:

$-h=-\frac{1}{2} g t^{2}1^{\text {st }} \text { ball }$
$\Rightarrow 122.5=\frac{1}{2} \times 9.8 t^{2}$
$\rightarrow t^{2}=25 \Rightarrow t=5\, s$
Another ball is dropped after $2$ second so it took only $(5-2)=3\, s$
$-122.5=-u(3)-\frac{1}{2} \times 9.8 \times 3^{2}$
$\Rightarrow 122.5=3 u+4.9 \times 9$
$\Rightarrow 3 u=78.4$
$\Rightarrow u=26.1\, s$