Viscosity

Sections 7.2 and 7.3 dealt with matter that is essentially static (a deformed solid returning to rest, a fluid sitting still in a container). This section turns to fluids that are actually MOVING. A moving fluid is a genuinely complex system: as it flows it possesses kinetic energy, potential energy and (as the later Bernoulli sections show) pressure energy simultaneously, and on top of all that, friction between its own layers -- viscous forces -- comes into play. To make the physics tractable, it helps to first imagine an IDEAL LIQUID: one that is completely incompressible (an infinite bulk modulus) and in which no shearing forces can be sustained at all (a coefficient of viscosity of exactly zero). Real fluids, however, do resist relative motion between their own layers -- most fluids offer some resistance to flow, exactly the way a frictional force acts wherever a solid moves across a surface, or wherever two fluids move relative to each other. This internal friction between the layers of a moving fluid is called VISCOSITY, formally defined as the property of a fluid that makes it oppose relative motion between its own layers. This section develops viscosity quantitatively, classifies flow as streamlined or turbulent using Reyno …