A plate of mass M remains in equilibrium in air when a bullets are fired per second on it. The mass of each bullet is m and it strikes the Late with speed v. If the coefficient of restitution is e, then {M >> m)

See fig. (8), The change in momentum of bullet=mev−(−mv) =mv(1+e) Given that n bullets strike per second. .'. Force exerted by the bullets =n[mv(1+e)] upwards   Now n[mv(1+e)]=Mg (∵F=Mg downwards ) so M=nmv(1+e)g