Question

# A metal plate of mass 200 gm is balanced in mid â€“ 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=9.8 m/s2)

A
20 m/s
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B
12.25 m/s
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C
10 m/s
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D
5 m/s
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Solution

## The correct option is B 12.25 m/sNumber of balls striking the metal plate per second =n=40 Force exerted by the balls per second on the metal plate, taking downward direction as positive F=nm(v−(−u))t=nm(v+u)t=nm(2u)t (∵ velocity of the ball remains same) As the plate is balanced in the mid-air it means weight of the metal plate is balanced by the force exerted the balls. ⇒0.2×9.8=40×2×10−3×2u1 ⇒u=12.25 m/s

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