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2.Motion in Straight Line
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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 ...... $m / s$
A
$49$
B
$55.5$
C
$26.1$
D
$9.8$
(AIIMS-2015)
Solution
(c)
$-h=-\frac{1}{2} g t^2$ ($1^{\text {st }} \text { ball }$)
$\Rightarrow 122.5=\frac{1}{2} \times 9.8 t^2$
$\Rightarrow t^2=25 \Rightarrow t=5 \,s$
Another ball is dropped after $2$ second so it took only $(5-2)=3 s$
$-122.5=-u(3)-\frac{1}{2} \times 9.8 \times 3^2$
$\Rightarrow 122.5=3 u+4.9 \times 9$
$\Rightarrow 3 u=78.4$
$\Rightarrow u=26.1 \,s$
Standard 11
Physics
