Let the force acting on the piston be $\mathrm{F}$ and the length of the cylinder be $l$. Then, work done $=F l$ Applying Bernoulli's theorem for points
$A$ and $B, p=\frac{1}{2} \rho v^{2}$ where $\rho$ is the density and $v$ is the velocity at point $B$. Now, force on the piston, $F=p A=\frac{1}{2} \rho v^{2} A$
where $A$ is the cross section area of piston. Also, discharge through the orifice during time interval $t=S v t$ and this is equal to the volume of the cylinder, i.e.,
From Eq. (1), (2) and (3) work done $=\frac{1}{2} \rho v^{2} A l=\frac{1}{2} \rho A \frac{V^{2}}{(S t)^{2}} l=\frac{1}{2} \rho V^{3} / S^{3} t^{2}$