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A man throws balls with same speed vertically upwards one after the other at an interval of 2sec. What should be the speed of throw so that more than two balls are in air at any time?
Explanation
Height attained by balls in 2sec is = $\frac{1}{2} \times 9.8 \times 4 = 19.6 m$ the same distance will be covered in 2 second (for descent) Time interval of throwing balls, remaining same. So, for two balls remaining in air, the time of ascent or descent must be greater than 2 seconds. Hence speed of balls must be greater than $19.6 m/\text{sec}$.