A ball is dropped from a bridge of 122.5 metre above a river. Aft… - Vidyadip

MCQ

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
$55.5$
✓
$26.1$
D
$9.8$

✓

Answer

Correct option: C.

$26.1$

c
(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 $(5-2)=3 s$

$-122.5=-u(3)-\frac{1}{2} \times 9.8 \times 3^2$

$\Rightarrow 122.5=3 u+4.9 \times 9$

$\Rightarrow 3 u=78.4$

$\Rightarrow u=26.1 \,s$

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