Hydrostatic balance

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The hydrostatic equilibrium ( hydro "water", statically "unmoved") is a mechanical equilibrium between gravity , which pulls a solid body down, and the static buoyancy , which tries to lift the body out of the liquid .

Viewed more generally, the hydrostatic equilibrium is always the balance between a directed force and a pressure gradient . Then convections can also be described using hydrostatic compensation.



All living beings with air pockets in their bodies (which applies to practically all bony fish ) must continuously readjust to maintain the hydrostatic balance. As the pressure increases with depth, trapped air is compressed as the pressure increases. The compression reduces the volume and thus the lift. The hydrostatic equilibrium therefore only ever exists for a certain depth and must be adjusted when the depth changes. Living beings with air pockets in their bodies are fish with their swim bladders and humans ( divers ) with their lungs or diving equipment. When divers are called this readjustment tare .


Diving ships ( submarines ) can absorb water by flooding diving tanks and thus submerge by increasing their (average) density from surface travel. By carefully blowing (with compressed air) such (rigid) tanks, their water content can be regulated and the submarine can be adjusted to the surrounding water in terms of density.

A diver fills the soft, flabby bladder of his buoyancy compensator (jacket) with air to the extent that - with an average lung filling - allows him to float at a certain depth without strenuous swimming movements.

However, the hydrostatic equilibrium achieved in this way is only unstable. Diving a little deeper compresses both the air bubble in the diving vest (and additionally in the neoprene (foam) suit) and the volume of air in a non-pressurized (open at the bottom) diving tank. The weight force becomes greater than the buoyancy force and pulls the boat or person further down, if not counteracted by blowing air into the tank or bladder. Conversely, when looking for a higher diving position, some of the then expanded air must be let out of the bladder. Finding this balance is trained as taring.

In Galileo thermometer float depending on the temperature (and therefore density) different balls.


In the case of larger celestial bodies such as planets or stars there is a layering that is caused by materials of different densities. For each layer boundary, an equilibrium is established between the attractive force of the underlying mass and the pressure generated by various physical processes, so that the rotation results in an approximately spherical shape ( see also reference ellipsoid ). For example, the Earth an inner ball that of iron and nickel existing earth's core , and overlying spherical shells ( mantle as well as the earth's crust ). The pressure can be applied by the gas pressure as well as by the radiation pressure in stars or by the degenerative pressure in white dwarfs or neutron stars .


In meteorology and atmospheric physics , the concept of the air parcel ( parcel method ) is used, which is sufficiently homogeneous to be treated like a solid in the atmosphere (to "swim"). The term static stability ( stratification stability ) of the atmosphere is also used for this.

See also


  • Dieter Meschede: Gerthsen Physics. 24th edition. Springer 2010, ISBN 978-3-642-12893-6 , section on pressure in static and liquid gases p. 101ff.

Individual evidence

  1. WA Magnitsky, WW Browar, BP Schimbirew: Textbook theory of the figure of the earth. Moscow 1961 (Russian) and Publishing House for Construction (East Berlin) 1964.