✓ Sources checked · 10 September 2026 · View verification
Liquids and gases are fluids. In a fluid at rest, pressure acts normally on surfaces and increases with depth in a gravitational field; moving flows need additional momentum and energy accounting.
By Matan · Updated
Pressure depends on depth, not container width
For uniform density, p = psurface + ρgh. This gives pressure at a point. The total force on a large wall requires summing the varying pressure over its area. Atmospheric pressure on opposite faces may cancel in the net load. [1]
Worked water force on a vertical panel
A vertical rectangular panel is 1.0 m wide and extends from a water surface to depth 2.0 m. Water density is 1000 kg/m³ and g = 9.8 m/s²; atmospheric pressure is the same on the dry side. Gauge pressure rises from zero to 19.6 kPa. Average gauge pressure is 9.8 kPa, giving horizontal force 19,600 N over the 2.0 m² area.
The load acts below the panel midpoint
For this triangular pressure distribution, the resultant acts two thirds of the depth below the surface: 1.33 m. Replacing the distributed pressure by a force at the geometric midpoint would give the wrong moment.
Flow pressure needs its assumptions
Bernoulli’s relation can connect pressure and speed for an appropriate steady ideal flow, but speed alone does not determine pressure everywhere. Hydraulics transmits pressure changes and trades piston travel for force. Compressible gases, changing density and unsteady flow require more complete models.
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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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Cite this article
Matan. “Fluids and Pressure.” Your Physicist. Originally published 2026-06-11; updated 2026-09-10. https://your-physicist.com/fluids-and-pressure/
Revision history
10 September 2026: Added integrated panel force and load position; qualified the universal fast-flow/low-pressure claim. Added references and a transparent verification record.
