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 = (dp/dt) where dp = change of momentum of water of mass dm striking  the ball a speed v during time dt. 

Since water falls dead after collision with the ping-pong ball
$\mathrm{\Delta p}=\mathrm{\Delta mv}\Rightarrow \mathrm{F}=\mathrm{v}\frac{\mathrm{\Delta m}}{\mathrm{\Delta t}}$upward on the ball 

Where $\frac{\mathrm{\Delta m}}{\mathrm{\Delta t}}$= rate of flow of water in the nozzle. 

Since the ball is in equilibrium
$\begin{array}{l}\mathrm{F}-\mathrm{mg}=0\Rightarrow \mathrm{F}=\mathrm{mg}\\ \Rightarrow =\mathrm{mg}\Rightarrow \frac{\mathrm{\Delta m}}{\mathrm{\Delta t}}=\frac{\mathrm{mg}}{\mathrm{v}}\end{array}$