Pressure may be defined as the perpendicular force exerted per unit area. A common unit of pressure used in aviation is the millibar. A millibar equals a force of 100 Newtons per square meter. In a fluid at rest the pressure at any given point is the same in all directions.
The animation below represents a sealed container of air at ambient pressure. The constant vibration (not shown) of the molecules exerts a force that is felt throughout the gas. The pressure at any given point is the same in all directions. For example at any point on the surface of the molecule highlighted a force of 101,300 Newtons per square meter can be considered to act upon a plane tangential to that point.
The pressure in a gas is affected by the volume, density and temperature of the gas. The next page will look at the relationship between pressure and volume.
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