It is assumed that plane surfaces in 2D will be parallel to the z axis (into the page). Therefore plane surfaces appear as a line with a given angle to the vertical axis. The plane could be any two dimensional shape, a triangle, circle, rectangle, or any 2D surface. At this point to figure out the [[Force|force]] due to [[Fluid Pressure|fluid pressure]] we simply: 1. Calculate $F$ 2. Calculate $(xc,yc)$ 3. In your free-body diagram plot $F$ at $(xc,yc)$, in the direction perpendicular to the plane. ![[hydrostatics_plane.png|center|500]]