Pressure
Pressure, in physics, force per unit area; that is, force divided by the area against which it is applied. A common unit of measurement for pressure is the pound-force per square inch (often called simply pound per square inch and abbreviated psi). The metric unit for pressure is the pascal, which equals one newton of force per square meter. (One psi equals approximately 6,895 pascals.) Since pressure depends on both area and the strength of a force, a given force can produce widely differing pressures. For example, a 10-pound force applied to an area of 1 square inch exerts a pressure of 10 psi. The same force applied to an area of 10 square inches exerts a pressure of 1 psi. Pressure is always a push at right angles to each point on a surface.
Atmospheric pressure, which is caused by gravity acting on the air that surrounds the earth, plays an important part in weather. Humans cannot live unprotected at pressures greatly above or below atmospheric pressure. Pressure also affects the temperature at which various substances boil. Water boils at higher temperatures when under high pressure. Pressure acting upon material deep in the earth is partially responsible for forming metamorphic rocks. In the ocean, pressure increases rapidly with depth.
In hydromechanics, the study of the effect of forces on fluids, several important laws relate to changes in and transmission of pressure. Bernoulli's Principle explains the reduction of pressure in moving streams of liquids and gases. Pascal's Law states that a change in pressure is transmitted equally to all parts of a confined fluid. Boyle's Law relates changes in pressure in a gas to changes in volume when the temperature remains constant.