Show that the system of hydrostatic forces acting on a submerged plane area A may be reduc...

Show that the system of hydrostatic forces acting on a submerged plane area A may be reduced to a force P at the centroid C of the area and two couples. The force P is perpendicular to the area and of magnitude  where γ is the specific weight of the liquid, and the couples are represented by vectors directed as shown and of magnitude  and  where  (see Sec. 9.8). Note that the couples are independent of the depth at which the area is submerged.