Question

A ball is dropped from the top of $80m$ high tower. If after $2\sec$ of fall the gravity $\left(g=10m/s^2\right)$ disappears, then time taken to reach the ground since the gravity disappeared is

• A. 2 s
• B. 3 s
• C. 4 s
• D. 5 s

Answer 1

Answer: (B)

Velocity acquired by the ball after $2s$ of drop is
$v=gt=10\times 2=20m/s$ Height fallen by the ball is
$h=\frac{1}{2}g{t}^2=\frac{1}{2}\times 10\times 4=20m$
After gravity disappears, speed is constant.
$\Rightarrow H-h=(v)\times t^{\prime }$
$\Rightarrow 80-20=(20)\times t^{\prime }$
$\Rightarrow 60=20t^{\prime }$ or $t^{\prime }3s.$