Hydrostatikk

Hydrostatikk, or hydrostatics, is the branch of fluid statics that studies fluids at rest under the influence of gravity. It analyzes how pressure is distributed inside a confined liquid and how objects behave when submerged in or resting on a liquid. The fundamental relation is the hydrostatic pressure equation: dP/dz = -ρ g, where P is pressure, z is the vertical coordinate, ρ is fluid density, and g is gravitational acceleration. For an incompressible liquid of constant density, the pressure increases with depth as P = P0 + ρ g h, with P0 the pressure at a reference level, typically the liquid surface. In open bodies of liquid, P0 equals atmospheric pressure.

A key principle is that pressure acts equally in all directions at a point (isotropy), which leads to buoyant forces on submerged bodies. Archimedes' principle states that the buoyant force equals the weight of the displaced fluid, explaining why objects float or sink and how stability depends on relative densities.

Hydrostatics also covers devices such as manometers and barometers, which measure liquid pressure and atmospheric pressure, respectively. The hydrostatic paradox notes that the pressure at the bottom of a vessel depends only on liquid height and density, not on total contained volume or vessel shape.

Applications span civil engineering (dams, water tanks, pressure pipes), naval architecture (buoyancy, hull design), and meteorology (pressure distribution in the atmosphere treated as a hydrostatic fluid under gravity). In many practical problems, liquids are treated as incompressible, though gas pressures require consideration of compressibility in the vertical direction.