Question

A body dropped from the top of a tower covers a distance $7x$ in the last second of its journey where $x$ is the distance covered in first second. How much time does it take to reach the ground (Take $g=10{m/s}^2$)?

• A. $3s$
• B. $4s$
• C. $5s$
• D. $6s$

Answer 1

The correct option is B $4s$

In first second, distance travelled $x=\frac{1}{2}\times g\times t^2=5m$
(for $g=10{m/s}^2$)
Distance travelled in last second $=7x=35m$
We know that $S_n=u+\frac{1}{2}a(2n-1)$
or $35=0+\frac{10}{2}\times (2n-1)$
$\Rightarrow n=4$
$\therefore t=4s$