Wednesday, 11 February 2015



Friction, also referred to as friction, is the inhibition of a movement occurring between contacting solid bodies or particles. We distinguish between external friction between contacting interfaces of solids and internal friction between adjacent particles in the deformation of fluids and solids. In physical models frictional forces are often neglected when they are relatively small and / or quantitatively elusive. With regard to the energy loss occurring in the friction (their conversion into heat with an increase in entropy) is referred to as the dissipation, particularly in the case of kinetic energy, but also in general terms for non-mechanical processes.

Instead of "friction force" and the term is used frictional resistance. With the scientific study of friction processes dealt tribology (friction teaching).

Types of friction

External friction

External friction is also called solid friction, because it occurs between the contact surfaces of the contacting solids. It is divided into static friction and kinetic friction, both of which are designated in honor of the physicist Charles Augustin de Coulomb as Coulomb friction. You do not always segregate on. You can also or alternately occur; For example, the stick-slip effect is a periodic transition between static and dynamic friction. On the application-related terms are rolling friction, boring friction and rope friction.

The friction force FR increases with normal force FN, often approximately linear and independent of the size of the contact surface (see Amontonssche laws):

F _ {\ mathrm R} <= \ mu \ cdot F _ {\ mathrm N}.

The friction coefficients μ are dependent on the nature of the surfaces. The coefficient of adhesion is generally greater than that for sliding. Its value is determined experimentally. The inequality comes from the fact that a frictional force can never encounter.


In many cases, adhesion between contacting bodies is desirable. Without friction of everyday life would not work. Furniture would not stay in place, parked vehicles on the road (the wheels blocked) could be propelled solely by the wind. You could not set foot "fixed" on the ground, all driven wheels would "spin", so do not allow traction. In industrial applications, a technically generated pressure between the contact surfaces is used except for the most acting weight force, for example by means of tensioned springs in a friction clutch.

Stick is a state of rest, so that neither wear nor energy losses occur. It is a combination of form fit on a small scale by roughness as structural variation 3rd to 5th order, would be destroyed in motion, and molecular adhesion Retail by molecular attraction forces, ie adhesion.

Sliding friction

Sliding friction occurs at the contact surfaces between bodies that move relative to each other. For some material combinations creep occurs, so that the friction force opposing the Amontonsschen law is a function of speed. The sliding friction is usually lower than the static friction force with the same normal force.