A ping-pong ball of mass m is floating in air by a jet of water emerging out of a nozzle. If the water strikes the ping-pong ball with a speed v and just after collision water falls dead, the rate of mass flow of water in the nozzle is equal to:

The impact force F = Δp / Δt where Δp = change of momentum of water of mass Δm striking the ball a speed v during time Δt . Since water falls dead after collision with the ping-pong ballΔp = Δmv ⇒  F = v (Δm /Δt) upward on the ballWhere Δm/Δt = rate of mass of water in the nozzle.Since the ball is in equilibrium.F - mg = 0  ⇒ F = mg ⇒  V . (Δm/Δt) = mg ⇒  Δm/Δt=mg/VHence (3) is the corerct choice