Question

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

• A. Any speed less than 19.6 m/s
• B. Only with speed 19.6 m/s
• C. More than 19.6 m/s
• D. At least 9.8 m/s

Answer 1

Answer: (C)

Time taken by ball to reach maximum height
$v = u - gT$
at maximum height, final speed is zero ie, $v = 0$
so, $u=gT$
or $T=u/g$
In $2s,\, u-2\times 9.8-19.6\, ms^{-1} $
If man throws the ball with velocity of $19.6\, ms^{-1} $ then after $2 s$ it will reach the maximum height. When he throws $2^{nd}$ ball, $1^{st}$ is at top. When he throws third ball, $1^{st}$ will come to ground and $2^{nd}$ will at the top. Therefore, only $2$ balls are in air. If he wants to keep more than $2$ balls in air he should throw the ball with a speed greater than $19.6\, ms^{-1}$