The phenomenon of dry friction (or Coulomb friction) occurs when the surfaces of two dry, unlubricated solids move past one another. The plots illustrate a model for dry friction: a mass attached to a spring with stiffness moves over a continuous belt traveling at constant speed , with the frictional force . The position of the mass is shown as a function of time and also plotted against . The effect of dry friction on machine operation is an important consideration in mechanical engineering.
The equation of motion is .
 D. W. Jordan and P. Smith, Nonlinear Ordinary Differential Equations, An Introduction for Scientists and Engineers, 4th ed., New York: Oxford University Press, 2007 pp. 32–34.