Fluid Mechanics

Centre of Pressure

The point on a submerged surface where the resultant hydrostatic force acts — always below the centroid.

Hydrostatic pressure increases with depth, so the resultant force on a submerged plane surface acts lower than its centroid. Measured along the plane from the free surface, its location is y_cp = y_c + I_c/(y_c·A), where I_c is the surface’s moment of inertia about its own centroidal axis.

For a vertical rectangle whose top is at the surface, the centre of pressure is two thirds of the way down. It matters whenever a gate is hinged or a wall can overturn, because the moment of the force depends on where it acts, not just how large it is.

How much of the FE Civil exam is Fluid Mechanics?