Question

A stone dropped from the top of a tower travels 15m in the last second of its motion. If $g=10m{s}^{-2}$ then the time of fall is

• A. $2s$
  
• B. $2.5s$
  
• C. $5s$
  
• D. $3s$
  

Answer 1

The correct option is A $2s$

Let 'n' be the last second of travel
$s=u+\frac{a}{2}(2n-1)$
($u$ is $0$)
$15=\frac{g}{2}(2n-1)$
$n=2$
2nd second is the last second of travel. Therefore time taken to reach the bottom is $2$.