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.