pressure, in thephysical sciences, the perpendicularforce per unit area, or thestress at a point within a confinedfluid. The pressure exerted on a floor by a 42-pound box the bottom of which has an area of 84 square inches is equal to the force divided by the area over which it is exerted; i.e., it is one-half pound per square inch. Theweight of theatmosphere pushing down on each unit area ofEarth’s surfaceconstitutesatmospheric pressure, which atsea level is about 15 pounds per square inch. InSI units, pressure is measured inpascals; onepascal equals onenewton per squaremetre. Atmospheric pressure is close to 100,000 pascals.

The pressure exerted by a confinedgas results from the average effect of the forces produced on the container walls by the rapid and continual bombardment of the huge number of gasmolecules. Absolute pressure of a gas orliquid is the total pressure it exerts, including the effect of atmospheric pressure. An absolute pressure of zero corresponds to empty space or a completevacuum.

Measurement of pressures by ordinarygauges on Earth, such as atire-pressure gauge, expresses pressure in excess of atmospheric. Thus, a tire gauge may indicate a pressure of 30 pounds (per square inch), the gauge pressure. The absolute pressure exerted by theair within the tire, including atmospheric pressure, is 45 pounds per square inch. Pressures less than atmospheric are negative gauge pressures that correspond to partial vacuums.

Britannica Quiz

Physics and Natural Law

Hydrostatic pressure is the stress, or pressure, exerted equally in all directions at points within a confined fluid (liquid or gas). It is the only stress possible in a fluid at rest.SeePascal’s principle.

Lithostatic pressure, the stress exerted on a body ofrock by surrounding rock, is a pressure in Earth’s crust somewhatanalogous to hydrostatic pressure in fluids. Lithostatic pressure increases with depth below Earth’s surface.