Question

A stone is dropped from a certain height which can reach the ground in $5s$. If the stone is stopped after $3s$ 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, $s=ut+\frac{1}{2}a{t}^2$
Distance covered in $5s$
$s=0+\frac{1}{2}\times 10\times 5^2=125m$
Distance covered in $3s$
$s=0+\frac{1}{2}\times 10\times 3^2=45m$
Distance to be covered $125-45=80m$
From, $s=ut+\frac{1}{2}a{t}^2$
$80=0+\frac{1}{2}\times 10t^2$
$t^2=\frac{80}{5}=16$
$t=4s$