For a liquid whose velocity potential changes from $\phi$ to $\phi^{\prime}$ owing to an impulsive pressure $p^*$ and impulsive forces $\mathrm{F}^*=-\nabla V^*$, show that
$$
p^*+\rho V^*+\rho\left(\phi^{\prime}-\phi\right)=\text { const. }
$$
Apply the equations of impulsive action to show that, if liquid be contained within a closed surface, the circulation and the vorticity cannot be altered by any impulse applied to the boundary.