In Physique one calls hydrostatic balance the state reached by a system when the forces of Gravitation are counterbalanced by a Gradient of Pression of opposite direction. One uses it in various fields: Meteorology, Oceanography physical, Astrophysical, etc

Meteorology and oceanography

To know the variation of pressure with altitude in the atmosphere or the depth in the Océan, one generally takes as assumption hydrostatic balance, that is to say that the variation of pressure (P) with the height/depth (Z) is proportional to gravity (G) and the density of the fluid:

$\backslash \; share\; P/\backslash \; share\; Z\; =\; -\; \backslash \; rho\; G$

This is not the case in the fast movements of Convection, as in the Orage S, but is checked rather well in the slower movements and with large scales: the synoptic scale.

Astrophysics

This concept is particularly useful in Astrophysique to classify the celestial objects. Thus the star S located on the principal sequence of the diagram of Hertzsprung-Russell are in a state of hydrostatic balance, stable in the face and temperature. This is allowed by the reactions of nuclear Fusion occurring in the middle of the star and whose radiative pressure, of direction opposed to the gravitational forces, counterbalances these last.

The concept of balance hydrostatic also makes it possible to determine if an celestial object is a Planet, a dwarf Planet or a smaller element of a solar system (Comet, Astéroïde, etc). According to the new definitions enacted in 2006 by the international astronomical Union, the dwarf planets and planets are objects having a sufficient gravity to maintain their own rigidity and to tolerate a hydrostatic balance, in a roughly spherical form.

References

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Robert McNamara | Cipaille | Trevor Morris | Orant | Albert-Small Armand