a cylindrical drum with no top of height 3 meters and radiius 2 meters is half filled with water and capped. if the drum is turned to a horizontal position, what is the force on the cap?

b. if the drum were completely filled before being capped and turned what would the force be on the cap?

we have to use some formula∫p.x.s(x)dx

from a to b where s(x) is the width of the object... I am confused as to how do I find the width of the object? i keep on thinking it is a 3D problem but in my book they make it seem like it is 2D.. the answer for a is 156 800/3, p is a constant = 9800 ..... and for b is 8*9800*pi

i actually dont understand anything. any help will do..