The velocity potential for a certain inviscid flow field is [ phi=-left(3 x^{2} y-y^{3} ight) ] where (phi) has the units of (mathrm{ft}^{2} / mathrm{s})

The velocity potential for a certain inviscid flow field is

\[ \phi=-\left(3 x^{2} y-y^{3}\right) \]

where \(\phi\) has the units of \(\mathrm{ft}^{2} / \mathrm{s}\) when \(x\) and \(y\) are in feet. Determine the pressure difference (in psi) between the points \((1,2)\) and \((4,4)\), where the coordinates are in feet, if the fluid is water and elevation changes are negligible.