A disc of mass `10 g` is kept floating horizontally by throwing `10"marbles"//"second"` against it from below.If the mass of each marble is `5 g` , the velocity with which marbles are striking the disc (the marbles strike the disc normally and rebound downward with the same speed)

Options

A. `3 m//s`
B. `2 m//s`
C. `1 m//s`
D. `4 m//s` in original direction

Answer

C

Explanation

Change in momentum of marble `= 2 mv`
`n`: number of marbles/second.
Force exerted by marbles `F = 2 n mv`
Weight of disc `W = Mg`
In equilibrium `F = W`
`2 n m v = Mg`
Here , `n = 10 , m = 5 g , M = 10 g , g = 10 m//s^(2)`
`2 xx 10 xx 5v = 10 xx 10 rArr v = 1 m//s`