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Buoyancy is the upward resultant of fluid pressure on an immersed body in ordinary hydrostatic conditions. For uniform density, its magnitude is the weight of fluid displaced.
By Matan · Updated
Weight, buoyancy and net force
Use Fb = ρfluidgVdisplaced. A floating body at rest with no other vertical support has Fb = mg, not Fb greater than mg. If buoyancy exceeds weight and other forces are absent, the body accelerates upward. [1]
Worked tethered block
A block has volume 0.010 m³ and mass 6.0 kg. Fully submerged in fresh water of density 1000 kg/m³, it receives 98 N buoyancy with g = 9.8 m/s². Its weight is 58.8 N, so a downward tether force of 39.2 N holds it stationary. Released and eventually floating at rest, it displaces 0.0060 m³: 60% of its volume.
Balloons use the same accounting
For a balloon, compare displaced-air weight with the combined weight of internal gas, envelope and payload. Heating gas does not remove the envelope’s weight. Ordinary airplane and bird flight primarily use aerodynamic lift, which is different from hydrostatic buoyancy.
Neutral force balance does not set velocity
Equal weight and buoyancy means zero net vertical force only if other forces are negligible. It does not by itself force an already moving body to stop. Drag, added fluid inertia and changing immersed volume affect motion after release; the tether calculation describes equilibrium.
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How this article was checked
Sources checked; calculations checked where shown. Main claims and model assumptions were checked against the references below. Example arithmetic and units were checked separately. Numerical examples are illustrative calculations, not experimental measurements.
Source checking and editing assisted by AI. No independent human scientific review is recorded for this revision. This record applies to the version dated 10 September 2026.
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Sources and further reading
Cite this article
Matan. “Buoyancy Explained.” Your Physicist. Originally published 2026-06-11; updated 2026-09-10. https://your-physicist.com/buoyancy-explained/
Revision history
10 September 2026: Corrected floating-force balance and aircraft buoyancy conflation; added tether force and submerged-fraction calculations. Added references and a transparent verification record.
