Pressure

Pressure is force distributed over an area, equal to normal force divided by the area it acts on. In fluids, it arises from momentum transfer by moving particles and acts in all directions at a point.

Normal force: A contact force perpendicular to the surface between interacting bodies. Pressure counts the component of force perpendicular to the surface.
Force: An interaction that changes or tends to change an object's motion. Pressure is defined from the force acting normal to a surface.
Barometer: An instrument that measures atmospheric pressure. Barometers turn atmospheric pressure into a measurable height or sensor reading.
Absolute pressure: Pressure measured relative to a perfect vacuum. It includes atmospheric pressure, unlike gauge pressure.
Pressure gradient: The spatial rate and direction of change in pressure. Pressure gradients drive fluid acceleration and flow.
Kinetic theory of gases: A model that explains gas properties through the motion and collisions of microscopic particles. Molecular collisions with container walls produce gas pressure.
Area: A measure of the extent of a two-dimensional surface. The same force produces less pressure when distributed over more area.
Manometer: An instrument that measures fluid pressure, often by comparing liquid-column heights. A manometer makes pressure differences visible through fluid displacement.
Partial pressure: The pressure a component gas in a mixture would exert if it alone occupied the volume. Gas mixtures assign pressure contributions to individual components.
Bernoulli's principle: A relation among pressure, speed, and height along a streamline in ideal steady flow. It links changing flow speed with changing pressure under specified conditions.

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