Problem $6(10$ points$)$: A long solid cylinder of radius $0\mathrm{.}8$ m hinged at point A is used as an automatic gate, as shown in below. When the water level reaches $5$ m $,$ the gate opens by tuming about the hinge at point A$.$ Determine $($a$)$ the hydrostatic force acting on the cylinder and the angle the resultant force is acting on the gate.