A circular plate of 3 m diameter is immersed vertically with its centroid at a depth of 20 m below water. Another similar plate is lying horizontally at a depth of 20 m. The total force on the horizontal plate will be:

Explanation:

A plate immersed vertically:

• The magnitude of hydrostatic force is given by F = ρgh̅A 
• Where ρ = Density of the fluid, h̅ = Depth of center of gravity of the surface from free liquid surface, A = Area of the surface

A plate immersed horizontally:

• The magnitude of hydrostatic force is given by F = ρgh̅A 
• Where ρ = Density of the fluid, h̅ = Depth of center of gravity of the surface from free liquid surface, A = Area of the surface
• Therefore, the hydrostatic force (total force) in both cases is same due to the same value of Depth of center of gravity of the surface from free liquid surface