What is hydrostatic pressure and equilibrium?

Hydrostatic equilibrium is the condition where external forces like gravity are balanced by an internal pressure-gradient force in a fluid or plastic solid at rest. This balance prevents planetary atmospheres from collapsing or diffusing, and it causes massive celestial bodies to form rounded shapes. In smaller-scale engineering contexts, hydrostatic pressure also plays a key role during the pressure testing of containment vessels.

What we know

• Hydrostatic equilibrium occurs when external forces, such as gravity, are balanced by a pressure-gradient force. [1, 2]
• Vertical pressure variation results from the pull of gravity on a fluid, where a decrease in elevation corresponds to a taller column of fluid weighing down. [3]
• The pressure-gradient force prevents Earth's gravity from collapsing the atmosphere into a thin shell, while gravity prevents the force from diffusing the atmosphere into space. [2]
• Hydrostatic pressure tests for pressure vessels usually use water because it is safer and does not expand explosively if a fracture occurs. [4]

Where to go next

• what caused itHow does gravity shape planets?
• how it workedWhat is vertical pressure variation?
• a case in pointHow are pressure vessels tested safely?
• compared withWhat is oncotic pressure in blood vessels?
• what followedWhat distinguishes dwarf planets in astrophysics?
• an unexpected connectionHow does fluid balance determine the shape of distant worlds?The same fluid mechanics that describe vertical pressure variations and containment vessels also explain why massive objects in space are rounded into spheroids.

Sources

• [1]hydrostatic equilibrium (Wikidata Q208641) · CC0 1.0
• [2]Hydrostatic equilibrium · CC BY-SA 4.0
• [3]Vertical pressure variation · CC BY-SA 4.0
• [4]Pressure vessel · CC BY-SA 4.0
• [5]Oncotic pressure · CC BY-SA 4.0