question_answer

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

A)  2s            

B)  3s

C)  4s         

D)  5s

Correct Answer: B

Solution :

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