Pressure in Liquids and the Atmosphere
Pressure is force spread over area. In a still fluid, pressure increases with depth and fluid density. Pressure differences explain upthrust; average object density helps predict floating, and air pressure falls with altitude.
What a learner can do afterwards
- Calculates pressure from force and area, and pressure at depth from depth, density and gravitational field strength
- Explains upthrust as the pressure difference between the bottom and top of a submerged object
- Predicts whether an object floats by comparing its average density with the liquid's, and explains why air pressure falls with altitude
1 · Read
Pressure spreads a force over an area. Divide force by area to get pressure in pascals, where one pascal is one newton per square metre. The same weight on a stiletto heel presses far harder than on a snowshoe. Pins and knives work by shrinking the area.
Dive deeper and the pressure climbs. The extra pressure from the liquid equals depth times density times gravitational field strength, so doubling the depth doubles it. Denser liquids pile it on faster than light ones. That is why a dam wall is thickest at its base.
Extra pressure from water at three depths, in kilopascals, with field strength 10 N/kg. Double the depth and the extra pressure doubles.
Lena holds a block under water. Water pushes on every face, but the bottom face sits deeper and feels more pressure than the top, leaving a net upward push called upthrust. To predict floating she compares densities: an object whose average density is less than the liquid's floats, and a denser one sinks.
Car tires use gauge pressure, measured against the air around us. A reading above that air is positive, and a reading below it, as inside a partial vacuum, is negative. Absolute pressure adds the atmosphere on top. Air itself thins with height, since less air piles above you, so mountain air presses less.
Pressure is force over area, and depth piles it higher.
The rewritten readings were written for this dataset.
More to read
Force over a contact area
Calculate pressure by dividing the force perpendicular to a surface by its area. Its unit is the pascal: 1 Pa = 1 N/m². Equal weights produce greater pressure through a stiletto heel than through a snowshoe.
Pins, knives and drawing pins concentrate force on small areas. Reducing contact area raises pressure without increasing force. Skis and wide tires spread force across larger areas, reducing pressure.
Why deeper liquid presses harder
A liquid's added pressure is density × gravitational field strength × depth. With density and gravity unchanged, twice the depth gives twice the added pressure. A denser liquid produces a greater increase.
This pressure difference causes upthrust. An object's lower surface experiences more pressure than its upper surface, giving an upward resultant force. Comparing average densities predicts whether it floats or sinks: lower than the liquid favours floating; higher favours sinking.
Pressure references and changing air pressure
Gauge pressure compares a reading with local atmospheric pressure. It is positive above that reference and negative below. Absolute pressure uses a vacuum reference and equals gauge pressure plus atmospheric pressure. Tire gauges report gauge pressure; weather maps show atmospheric pressure on an absolute scale.
Higher altitudes have thinner air and less atmosphere overhead, so air pressure falls. A stationary barometer also detects pressure changes from passing weather systems. Those changes differ from the effect of gaining height.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.