Question

A stone is dropped from the ${25}^{th}$ storey of a multi-storeyed building and reaches the ground in $5s$. In the first second, it passes through how many storeys of the building? $(g=10m/s^2)$.

• A. 1
• B. 2
• C. 3
• D. None of these

Answer 1

The correct option is A 1

Suppose h be the height of each storey.
By using second equation of motion, we know that
$s=ut+\frac{1}{2}a{t}^2$
As the stone is at rest initially
$\Rightarrow u=0,a=g=10m/s^2$
$\therefore 25h=0+\frac{1}{2}\times 10\times t^2$
$25h=\frac{1}{2}\times 10\times 5^2\text{or}h=\text{5 m}$
In first second, let the stone passes through $n$ storeys.
So, $n\times 5=\frac{1}{2}\times 10(1{)}^2\Rightarrow n=1$