A) Only with speed 19.6 m/s
B) More than 19.6 m/s
C) At least 9.8 m/s
D) Any speed less than 19.6 m/s.
Correct Answer: B
Solution :
Height attained by balls in 2 sec is \[=\frac{1}{2}\times 9.8\times 4=19.6\text{ m}\] 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 \[19.6\text{ }m/sec\].
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