Question

A body is dropped from the roof of a multi-storeyed building. It passes the ceiling of the ${15}^{th}$ storey at a speed of $20m/s$ . If the height of each storey is $4m$, the number of storeys in the building is (take $g=10m/s^2$ and neglect air resistance)

• A. 20
• B. 25
• C. 30
• D. 35

Answer 1

Answer: (A)

20

$v=u+gt$
$\Rightarrow 20=10t$

$\Rightarrow t=2$
$S=ut+\frac{1}{2}g{t}^2=\frac{1}{2}\times 10\times 2^2=20$
$\Rightarrow$ it has travelled $5$ storeys + it has $15$ storeys below
$\Rightarrow$ There are total 20 storeys.