A man throws balls with the same speed vertically upwards one after the other at an interval of 2 seconds.…
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?
more than $19.6 \mathrm{~m} / \mathrm{s}$
at least $9.8 \mathrm{~m} / \mathrm{s}$
any speed less than $19.6 \mathrm{~m} / \mathrm{s}$
only with speed $19.6 \mathrm{~m} / \mathrm{s}$
Solution
From question Interval of ball thrown $=2 \mathrm{sec}$
If we want that minimum three balls remain in air then time of flight of first ball must be greater than 4 sec.
$\begin{aligned}
& T > 4 \mathrm{sec} \\
& \therefore \frac{2 u}{g} > 4 \mathrm{sec} \\
& \Rightarrow \quad u > 19.6 \mathrm{~m} / \mathrm{s}
\end{aligned}$