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 $5m$, then the number of balls thrown per minute is $\left(g=10m{s}^{-2}\right)$

• A. 120
• B. 80
• C. 60
• D. 40

Answer 1

Answer: (C)

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