Fluid is any substance that flows easily under pressure. All liquids and gases are fluids. Water at ordinary temperature is a liquid fluid. Air is a gaseous fluid. A liquid does not significantly change volume under increasing pressure. A gas, however, readily changes its volume by expanding or contracting to fill any container in which it is placed.
Gases are compressible fluids, and liquids are nearly incompressible fluids. For example, increasing the pressure on liquid water to 1000 times normal air pressure will decrease the water’s volume and increase its density by only about 3 percent (see Density ). For a gas, by contrast, doubling the pressure will decrease the gas’s volume by one half and double its density.
A perfect fluid is frictionless–that is, it offers no resistance to flow except that of inertia (see Inertia ). An elastic fluid has greater forces resisting changes to size or shape than forces resisting flow. A thicker, viscous fluid, such as molasses, is slow-flowing because of the fluid’s internal friction.
