Question

The upper and lower edges of a square lamina of length 4 m are at a depths of 1 m and 3 m respectively in water. What will be the total pressure (in kN) on the lamina?

(a) 156.96

(b) 235.44

(c) 313.92

(d) 392.4

The question was posed to me in quiz.

My doubt is from Hydrostatic Force on Plane Area in section Hydrostatic Forces on Surfaces of Fluid Mechanics

Answer 1

The correct option is (c) 313.92

To elaborate: Total liquid pressure on the lamina = F = γyA, where γ = specific weight of the liquid, y = depth of centroid of the lamina from the free surface, A= area of the centroid. Now, γ = 9:81 * 0.9 * 10^3 N / m^3; y = 1 + 3-1 / 2 = 2m, A = 4 * 4 = 16 m^2. Hence, F = 313.92 kN.