Slide 10 of 12
Notes:
Although it may not be obvious just how, both of these simple mathematical “laws” for how viscous friction impedes flow were derived by applying the definition of viscosity on the previous slide.
Stoke’s Law, although it applies only to spheres moving with irrotational flow through a viscous fluid, is representative of the viscous frictional force on any “streamlined” object moving through a fluid. In the absence of turbulence, viscous friction is always opposite to velocity and proportional to the product of speed, viscosity and linear dimension; the proportionality constant depends on shape.
For cars travelling at low speeds, air resistance is proportional to speed times viscosity. To get good mileage, drive slow in a small, streamlined car. Higher speeds lead to increasing turbulence and much stronger increase of air resistance with speed.
