A fluid is anything that flows: liquids and gases. Pressure is force per area, and in a fluid at rest it pushes equally in every direction. It grows with depth, because every layer carries the weight of the fluid above it: P = P₀ + ρgh, where ρ (rho) is density, g is 9.81 m/s², h the depth and P₀ the pressure at the surface.
10 m of water adds 1000 × 9.81 × 10 = 98,100 Pa, about 1 atm. A diver at 10 m feels about 2 atm in total. That's why tank walls and dams are thicker at the bottom, and why pumps have to lift against the head of liquid they push into.
Gauges usually read gauge pressure, how much above the atmosphere: a tyre at "2 bar" is really about 3 bar absolute. Absolute pressure = gauge + atmospheric (about 101 kPa). Gas law and vapor pressure sums need absolute pressure. A vacuum is a pressure below the atmosphere.
Engineers often give a pressure as a head: the height of liquid it would hold up, h = P ÷ (ρg). One bar is about 10.2 m of water. A manometer measures a pressure difference the same way: a U-tube of liquid whose two levels differ by h shows ΔP = ρgh.
Liquids barely compress, so their density stays constant. Gases compress a lot, and their density follows the ideal gas law; for small pressure changes, though, you can often treat them as constant too.
What gauge pressure, in kPa, is at the bottom of a 5 m deep water tank? (ρ = 1000 kg/m³, g = 9.81 m/s²) Round to one decimal.
| Hydrostatic | P = P₀ + ρgh |
|---|---|
| 10 m of water | ≈ 98 kPa ≈ 1 atm |
| Absolute | gauge + atmospheric (≈ 101 kPa) |
| Head | h = P ÷ (ρg) |
| Manometer | ΔP = ρgh |