A metal plate weighing 200g is balanced in mid air by throwing 40 balls per second vertically upwords from below. After collision balls comes out with the same speed. What is speed with which balls strike the plate? (Given mass of each ball 200g)

in this case the plate will be suspended only if

weight of the plate = force exerted by 40 balls per second

now,

weight of plate, W = Mg

force exerted by 40 balls per seconds, F = 40 x ma = 40 x mv/t = 40 x mv  [as t = 1s]

so,

Mg = mv

here,

mass of plate, M = 200g = 0.2 kg

mass of ball = 200g = 0.2 kg

so,

0.2 x 10 = 40 x  0.2 x v

or

v = 10/40

thus, the velocity of each ball will be

v = 0.25 m/s

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