# Buoyancy Buoyancy is the upward [[Force]] a fluid exerts on anything immersed in it, equal to the weight of the fluid displaced. Archimedes' principle is the whole of it, and the reason it works is [[Hydrostatics|hydrostatic]]: pressure rises with depth, so the push on the bottom of an object exceeds the push on its top. Whether the object rises or sinks is then a straight comparison of [[Density]] against the fluid's. ## On this spine Buoyancy is why [[Helium]] and [[Hydrogen]] are lifting gases and why a balloon is a [[Density]] argument rather than a chemical one. It drives natural convection wherever [[Heat_transfer|heating]] makes a fluid lighter, which is the engine of [[Rayleigh–Bénard_convection]] and of every thermal plume in the [[Atmosphere_of_Earth|atmosphere]]. Stratification — heavy fluid resting under light — is buoyancy holding a layer together against shear, and the criterion for whether it breaks is what makes a [[Kelvin–Helmholtz_instability]] possible or not. In [[Two-phase_flow]] it separates gas from liquid and decides which regime a pipe adopts. **Reads with:** *All Things Flow: Fluid Mechanics for the Natural Sciences (2019)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/all-things-flow-fluid-mechanics-for-the-natural-sciences) · [download](https://blogs.oregonstate.edu/salty/all-things-flow-fluid-mechanics-for-the-natural-sciences/). Seeded from the Thury Hydrodynamics Apex Spine book shelf. **On the spine:** [[Hydrostatics]] · [[Density]] · [[Rayleigh–Bénard_convection]] · [[WT!Thury_Hydrodynamics_Compendium]]. <!-- COMPENDIUMLINK:BEGIN g19 — generated from _registry/plans/THURY_COMPENDIUM_SECTIONS.md; do not hand-edit inside --> *Linked from the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]], section 2, Fluids at rest.* <!-- COMPENDIUMLINK:END --> <!-- SPINEPATH:BEGIN g20 — shortest chain of Wikipedia links between local articles to a Compendium Main article; do not hand-edit inside --> *Connected to the Apex Spine:* Buoyancy → [[Fluid_dynamics|Fluid dynamics]] — [[WT!Thury_Hydrodynamics_Compendium|Compendium]] section 1, *Fluid dynamics*. <!-- SPINEPATH:END --> <!-- THURYSIM:BEGIN g21 — Thury Compendium microsim (framework build, specs/variants/Buoyancy.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Buoyancy* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/thury/Buoyancy.html" data-title="Buoyancy"></div> *Built from `MICROSIM_GUIDE/specs/variants/Buoyancy.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.* <!-- THURYSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Buoyancy) : [Wikitube](https://en.wikitube.io/wiki/Buoyancy) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Physics]]. --- *Apex Spine wave · 2026-09-10 · stub · microsim layer deferred.*