Perpendicular force per unit area, or stress at a point within a confined fluid. A solid object exerts pressure on a floor equal to its weight divided by the area of contact. The weight of the Earth's atmosphere on the surface constitutes atmospheric pressure, which varies from place to place but always decreases with altitude. The pressure exerted by a confined gas results from the average effect of the forces produced on the walls of the container by the continual, numerous collisions by gas molecules. Hydrostatic pressure is the stress, or pressure, exerted equally in all directions at points within a confined fluid. Lithostatic pressure is the stress exerted on a body of rock in the Earth's crust by surrounding rock, which increases with depth below the surface. The SI unit of pressure is the pascal (Pa), which is equal to one newton of force per square metre.

This entry comes from Encyclopædia Britannica Concise.
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