Question

A very large number of balls are thrown vertically upwards in quick succession in such a way that the next ball is thrown when the previous one is at the maximum height. If the maximum height is $5 m$, then the number of balls thrown per minute is $\left(g=10\, m \,s ^{-2}\right)$

• A. 120
• B. 80
• C. 60
• D. 40

Answer 1

Answer: (C)

Given that maximum height $=5 m , g=10 m s ^{-2}$
Now, $v^{2}=u^{2}-2 g s$
or $ 0=u^{2}-2 \times 10 \times 5=u^{2}-100$ or
$u^{2}=100$
$\therefore u=10 m s ^{-1}$
Further, at maximum height, $v=0$
$\therefore 0=u-g t=10-10 \times t$ or $t=1 s$
It means that each ball is thrown after 1 s. Therefore, number of balls thrown per minute $=60$.