Question

A ball dropped from the $9^{\text {th }}$ storey of a multi-storeyed building reaches the ground in $3$ second. In the first second of its free fall, it passes through $n$ storeys, where $n$ is equal to(Take $\left.g=10 m s ^{-2}\right)$

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

Answer: (A)

Let the height of each storey be $y$.
$\Rightarrow $ 9th storey's height $=9 y$ The ball is dropped
$\Rightarrow \,u\, =\, 0$
$s=u t+\frac{1}{2} a t^{2}$
$9 y=(0)(3 \,s )+\frac{1}{2}\left(10\, ms ^{-2}\right)(3 s )^{2,} 9 y=\frac{10 \times 9}{2}$
$y=5 m$
In $1 s ,$ the ball travels, $s=\frac{1}{2}\left(10\, ms ^{-2}\right)(1\, s )^{2}=5 m$
Thus in 1 s, the ball travels 1 storey.