3if u want the net force on the lamina ................................
let us divide into rods ..................................
now assuming the top of the lamina is just on the surface ...................
A = 1/2 Hb ............
A/H = b/2 .........[b=base length].....
A/H2x = y ............[as we divide into segments dx in thickness and x is mesured top]
[ y = 1/2 length of the rod at that x ]
pressure at depth x = Pgx [P =density]
Pgx2(dx)(2A/H2) = dF ///////
(2/3)PgAH = F ........................
[i think by thrust u want the net force on the lamina viashnavi .....]
