A metal plate of mass $200 g m$ is balanced in mid–air by throwing $40$ balls/sec, each of mass $2 g m$ , 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 $g = 10 m / s^{2}$ )

20 m / s

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12.5 m / s

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10 m / s

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5 m / s

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Solution

The correct option is B $12.5 m / s$
Considering downward as positive,

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

$\Rightarrow v = 12.5 m / s$

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A metal plate of mass 200 gm is balanced in mid–air by throwing 40 balls/sec, each of mass 2 gm, 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=10 m/s2)

The correct option is B 12.5 m/sConsidering downward as positive, F=mg=nm(v−(−u))t=nm(v+u)t ⇒0.2×10=40×2×10−3×2v1 (v=u, given that balls rebound with the same speed) ⇒v=12.5 m/s

A metal plate of mass $200 g m$ is balanced in mid–air by throwing $40$ balls/sec, each of mass $2 g m$ , 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 $g = 10 m / s^{2}$ )