Question

A metal plate of mass $200gm$ is balanced in mid–air by throwing $40$ balls/sec, each of mass $2gm$, vertically upwards from below. The balls rebound with the same speed with which they strike the plate. Find the velocity with which the balls strike the plate. (Take $g=10m/s^2$)

• A. $20m/s$
• B. $12.5m/s$
• C. $10m/s$
• D. $5m/s$

Answer 1

The correct option is B $12.5m/s$

Considering downward as positive,

$F=mg=\frac{nm(v-(-u\left)\right)}{t}=\frac{nm(v+u)}{t}$
$\Rightarrow 0.2\times 10=\frac{40\times 2\times {10}^{-3}\times 2v}{1}$
($v=u$, given that balls rebound with the same speed)

$\Rightarrow v=12.5m/s$