Vertical hydrostatic force acting upwards on a curved surface?

[theBEAST]

Homework Statement

I am only confused about F_y.

The Attempt at a Solution

I don't understand why in the equation F_y = ρgh_cA, the h_c is 5m. Why do they take the centroid distance to be the very bottom?

 

[haruspex]

They're analysing the forces on a particular region of the water, treated as a block. It's the region lying 'under' the cylinder, i.e. it's bounded by a vertical line through point A, the floor of the chamber, and a quadrant of the cylinder. Fy is the vertically upward force on this block from the floor of the chamber. It must be equal and opposite to the force the block exerts on the floor. The pressure there is the pressure at 5m depth.