Question

A man throws balls with same speed vertically upwards one after the other at an interval of $2 \,s$. What should be the speed of throw so that more than two balls are in the sky at any time ?

• A. Only with speed $19.6 \,ms^{-1}$
• B. More than $19.6\,ms^{-1}$
• C. At least $9.8 \,ms^{-1}$
• D. Any speed less than $19.6 \,ms^{-1}$

Answer 1

Answer: (B)

More than $19.6\,ms^{-1}$

Answer 2

Interval of ball thrown $= 2 \,s$ If we want that minimum three (more than two) balls remain in air then time of flight of first ball must be greater than $4\, s$. $T > 4\,s$ $\Rightarrow \frac{2u}{g} > 4s$ $\Rightarrow u > 19 .6 \,m\,s^{-1}$