# Friction

friction is a physical Kraft, which works against itself a relative motion between two affecting bodies.

Friction forces are first practically described. Example: humans sit on a Holzbrett on the soil. The Holzbrett is to be laterally pulled:

A senkrechte Kraft(Weight of humans and wood) has an appropriate maximum horizontal friction force, which holds the block in its position (“static friction”). These retaining Kraft usually expressed as relationship to the Gewichtskraft. E.G. meant “μ = 0.2” that a Gewichtskraft from a massby 100 kg a Kraft according to 20 kg for lateral pulling requires. That applies to Holzbrett and floor. With less than 20 kg lateral course the block does not move, due to the static friction. Now the block goes with higher laterallyTraction power in sliding over, then lowers in all rule Kraft, which are necessary for maintaining the movement: the sliding friction has mostly a lower coefficient of friction than the static friction. Vulgo: with 12 or 15 kg of lateral “traction power” then that remainsBoard including load (humans) in motion: μ directly 0,12 or 0,15. These proportionality factors or coefficient μ depend on both materials and possible materials between them.

“Bitumen/rubber” is the classical mating with vehicles. “Friction wheel/flint” is oneother friction mating: with lighters. “Crankshaft/sliding bearing/with oil between them” e.g. is. in the engine technology a description of friction.

In the broader sense liquids and gases friction forces also moved experience.

Friction belongs to the innumerable physical terms, metaphorically also in thatEveryday life language to be used (it gave a Reiberei between them; the administrative expirations bring friction losses with itself).

The technical field of activity in mechanical engineering, which scientifically busily with the lubricating of different mechanical components, a whose goal is the decrease of the friction, one calls oneselfas tribology. It is concerned among other things with the lubrication of camps, guidance, transmissions and engines.

## overview

friction depends on material properties of the rubbing bodies; physical statements about friction are therefore less generaland more inaccurately, than one is used to it from other physical regularities. The derivation that Basic Law of the mechanics was only possible at all, by neglecting friction.

Nontheless friction is a basic fact of our world: without friction one could itself neitherthe shoes zuknoten, still articles with nails or screws fasten.

Tribologi system

statements to the friction can be never made for a body or a material alone, in addition the tribology always regards a so-called. Tribosystem, consisting of base, Gegenkörper, Zwischenstoffand environment medium. With a ball bearing the bush of the bases is, the balls is Gegenkörper, the oil is the Zwischenstoff and air is the environment medium. Become the material properties of the media, the material and form characteristics of the bodies and the surface properties thatBody regards.

The distinction between the static friction and all remaining forms of the friction is fundamental; it will hold the opinion that static friction should not better at all be called friction. With exception of the static friction each friction causes Dissipation: it brakesthe relative motion of the bodies involved, converts mechanical energy into warmth and produces by it entropy. Friction can produce also electrical tension (triboelectricity).

With the friction between solid surfaces one differentiates depending upon geometry between sliding friction, rolling friction, Wälzreibung andDrilling friction (see below in the article). In the technology one uses lubrication, in order to lower the friction; according to whether the surfaces moved against each other are separate by a complete or incomplete liquid film, friction of a liquid or boundary friction can be present.

If a lubricant film, oneother liquid or a gas (generalized: a fluid) at a solid surface along flows, this current by friction is obstructed: the fluid is braked, if the current will not keep upright by a difference of pressure. This friction hangs less of thatCondition of the wall, when rather from the cross section of the current off, because the Dissipation is not on the boundary surface between fluid and wall limits, but takes place as internal friction (rheology) between different layers of the fluid, those depending upon proximityto the wall differently fast flow.

Relative to a fluid moved bodies experiences this friction as flow resistance (own article; for formulas and information in more detail see there). It experiences a Kraft, those its speed of v is against-arranged and thosewith laminar current (Stokesreibung) proportionally to v, with turbulent current (Newton friction) proportional to v 2 is. A body can experience flow resistance and dry friction at the same time: to the energy consumption of cars both the air turbulence and the rolling friction of the tires contribute.

From friction wear results. The verschleissmechanismen adhesion, abrasion, deformation and impulse oxidation work.

## friction between solids

sliding a solid body along another knows on the one hand by molecular attraction (adhesion) of the contact areas or its mechanical clasping(similarly filing) to be obstructed.

### general

if the relative velocity< math> \ vec {v} _ {\ rm rel} = \ vec {v} _1 - \ vec {v} _2< /math> between body 1 and body 2 at the edge contact zero are not equal, rub the bodies against this edge contact. For on bodies1 working frictional force [itex] \ vec {F} _ {\ rm g 1}< /math> f applies}

< _> {\ rm for g 1} = - \ mu_ {\ rm R after coulomb math \ vec {} |\ vec {F} _ {\ rm N}| \ frac {\ vec {v} _ {\ rm rel}} {|\ vec {v} _ {\ rm rel}|},< /math>

whereby [itex] \ vec {F} _ {\ rm N}< /math> the normal force (perpendicular to the affecting level at the edge contact) is and [itex] \ mu_ {\ rm R}< /math> coefficient of friction or coefficient of friction one calls. Forfrictional force the affecting body two applies accordingly

< math> \ vec {for F} _ {\ rm R2} = - \ vec {F} _ {\ rm g 1} = \ mu_ {\ rm R} |\ vec {F} _ {\ rm N}| \ frac {\ vec {v} _ {\ rm rel}} {|\ vec {v} _ {\ rm rel}|}. [/itex]

The modelling of the friction after this law is a rough approximation, however for technical problems is frequently used.

## static friction

### rolling friction

rolling friction (or rolling friction) develops, if a body on a document rolls. If the static friction between bodies and document is larger than the sum of the remaining in the leverageon the body, then the body without slip rolls working forces, and it affects pure rolling friction it; with sliding slip sliding friction portions are added.

Details in the article rolling friction.

### Wälzreibung

stepping sliding and rolling friction at the same time up, marksone this combination as Wälzreibung (see also rolling). The sliding friction offers a slighter resistance as the static friction however a larger than the rolling friction thus applies: Sliding friction< static friction sliding friction> rolling friction

### drilling friction

drilling friction develops, if itself a ball overthe vertical axle on a horizontal level turns. It is an equilibrium between friction resistance and torque T.
Coefficient of the drilling friction:

[itex] N_ {\ mathrm {Bo}} = \ frac {T} {F_N}< /math> in cm

## friction in the lubrication technology

### dry friction

with that The surfaces sliding one on the other touch themselves dry friction. Surface increases are gradened (abrasion or wear). During unfavorable combination of material and large surface pressure the surfaces weld with one another (adhesion). Dry friction arises for example, if no lubricant is used, or the lubricationfailed.

### boundary friction

the boundary friction can occur when insufficient lubrication or at the beginning of the movement of two friction partners with lubrication. The sliding surfaces touch themselves punctually. Friction force and wear are smaller than higher during the dry friction, but thanduring the friction of a liquid. This condition should be avoided in continuous operation, is however in the technology occasionally unavoidable.

### friction of a liquid

the friction of a liquid arises if between the sliding surfaces a permanent lubricating film forms. Typical lubricants are oils,Water in addition, gases (see air bearings). The sliding surfaces are completely from each other separated. The developing friction is based on the fact that the lubricant molecules slide one on the other. These shearing stresses lead to a rise in temperature of the lubricant. This must be exhausted in suitable way.

Friction of a liquid is the desired condition in camps and guidance, if service life, high running speed and high load are needed.

The transition from the boundary friction to the friction of a liquid is represented by the Stribeck curve. The friction of a liquid is proportional with laminar current to the speed of v, with more turbulentCurrent proportionally to v 2.

## internal friction

internal friction is an energy consumption with movement of the atoms and/or. Molecules of a material against each other, for example with currents within an oil. Outside forces can do like the force of gravityeach liquid particle affect and differences of pressure can cause accelerations. Friction forces cause the tenacity of materials and/or. the viscosity in liquids. For each liquid particle must itself the outside forces, the thrust forces, which hold friction forces and the forces of inertia the equilibrium.

The insideFriction is accessible with the means of the statistic physics of completely different and much more precise description, than the friction between careless solid surfaces. Differently than in the mechanics, in the friction so long as, is internal friction is possible neglected inthe standard theory of hydrodynamics - which Navier Stokes equations - contain firmly.

The rheology is concerned with friction in complex liquids, for example polymers and dispersions, to whose description the linear Navier Stokes equations are not sufficient. Nonlinearly also the friction is, those with deformation in solids arises.

### gas friction

during the gas friction concerns it for example the air resistance of a vehicle. Gas friction is particularly used in some cases:

• Parachutes, thus from the free case controlled sinking airbrakes
• at the airplane become, around the outline of the current to force
• throttles in gas lines for the delimitation of the flow rate
• producing Verdichtungswärme, e.g. to the compression ignition in diesel engines and turbines (subordinated effect, there the rise in temperature when compression by decrease of the thermal capacity of the gas develops.)

The gas friction is approximately proportional to the square of the flow rate v of the gas.