Question

A man throws balls with the same speed vertically upwards one after the other at an interval of $2$ seconds. What should be the speed of the throw so that more than two balls are in the sky at any time (Given $g=9.8{\text{m/s}}^2)$

• A. At least $0.8\text{m/s}$
• B. Any speed less than $19.6\text{m/s}$
• C. Only with speed $19.6\text{m/s}$
• D. More than $19.6\text{m/s}$

Answer 1

The correct option is D More than $19.6\text{m/s}$

Interval of ball throw $=2\text{sec}$
If we want that minimum three (more than two) ball remain in air, then time of flight of first ball must be greater than $4\text{sec}$.
$T>4\text{sec}$
$\frac{2u}{g}>4\text{sec}\Rightarrow u>19.6\text{m/s}$
for $u=19.6$.
First ball will just strike the ground (in sky)
Second ball will be at highest point (in sky)
Third ball will be at point of projection or at ground (not in sky)