Question

A stone is dropped from a certain height which can reach the ground in $5\,s$. If the stone is stopped after $3\, s$ of its fall and then allowed to fall again, then the time taken by the stone to reach the ground for die remaining distance is

• A. 2 s
• B. 3 s
• C. 4 s
• D. None of these

Answer 1

Answer: (C)

From, $ s=ut+\frac{1}{2}at^{2} $
Distance covered in $ 5s $
$ s=0+\frac{1}{2}\times 10\times 5^{2}=125\,m $
Distance covered in $ 3\,s $
$ s=0+\frac{1}{2}\times 10\times 3^{2}=45\,m $
Distance to be covered $ 125-45=80\,m $
From, $ s=ut+\frac{1}{2}at^{2} $
$ 80=0+\frac{1}{2}\times 10\,t^{2} $
$ t^{2}=\frac{80}{5}=16 $
$ t=4\,s $