Hello, I'm trying to go through an example problem in a fluid dynamics textbook, and I'm having trouble understanding some of their logic. The problem deals with a solid cylinder of radius 0.8 m hinged at its midpoint that opens when the water it holds back reaches 5 m, and it is asking for the hydro-static forces acting on the cylinder, where the center of the cylinder is 4.2 m below the surface (see attached picture). Their solution... For the horizontal force: Fx = FH = ρghcA = (1000 kg/m3) (9.81 m/s2) [4.2 m + (0.8 m/2)] (0.8 m*1 m) Now, I understand it up until the last part that deals with the area. Firstly, the 1 m length is never mentioned, is this just a typical assumption? Also (probably me missing something very basic here) but what area is radius * length? For Fy they use the same area component, but when they calculate the weight they use the formula: W = ρgV = ρg(R2 - πR2/4)(1) = (1000 kg/m3) (9.81 m/s2) (0.8 m)2 (1-π/4) (1 m) And again, my question is, what area are they using (multiplied by 1 m) to calculate the volume? Thanks in advance!