where nII and nIII are the unit normal vectors on the plate in Regions II and III, respectively. The unit normal vectors are taken positive pointing out of the fluid. In a three-dimensional model, F3 is the vertical force per unit length (into the page) of the plate. Recall also that we do not allow the top surface of the plate to become dry and, therefore, that กกกกกกกกก in (46) and (47) is always the water pressure.
The overturning moment due to the vertical force is calculated as