Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.^[2] Types of friction include dry, fluid, lubricated, skin, and internal.

For other uses, see Friction (disambiguation).

Friction can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components.

Friction is a non-conservative force – work done against friction is path dependent. In the presence of friction, some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved. Friction is not itself a fundamental force. Dry friction arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions makes the calculation of friction from first principles difficult and it is often easier to use empirical methods for analysis and the development of theory.

Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction ("") between non-moving surfaces, and kinetic friction between moving surfaces. With the exception of atomic or molecular friction, dry friction generally arises from the interaction of surface features, known as asperities (see Figure).

stiction

Fluid friction describes the friction between layers of a fluid that are moving relative to each other.^[3]^[4]

viscous

There are several types of friction:

First Law: The force of friction is directly proportional to the applied load.

Amontons'

Amontons' Second Law: The force of friction is independent of the apparent area of contact.

's Law of Friction: Kinetic friction is independent of the sliding velocity.

Coulomb

The elementary property of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:

$F_{\mathrm {f} }$ is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force.

$\mu$ is the coefficient of friction, which is an empirical property of the contacting materials,

$F_{\mathrm {n} }$ is the exerted by each surface on the other, directed perpendicular (normal) to the surface.

normal force

Radiation friction

As a consequence of light pressure, Einstein^[69] in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter. He wrote, "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backward-acting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."

Reduction

Devices

Devices such as wheels, ball bearings, roller bearings, and air cushion or other types of fluid bearings can change sliding friction into a much smaller type of rolling friction.

Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used in low friction bearings. They are especially useful because the coefficient of friction falls with increasing imposed load.^[73] For improved wear resistance, very high molecular weight grades are usually specified for heavy duty or critical bearings.

Lubricants

A common way to reduce friction is by using a lubricant, such as oil, water, or grease, which is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Lubricant technology is when lubricants are mixed with the application of science, especially to industrial or commercial objectives.

Superlubricity, a recently discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels. A very small amount of frictional energy would still be dissipated.

Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a lubricant.

Another way to reduce friction between two parts is to superimpose micro-scale vibration to one of the parts. This can be sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise, known as dither.

$\mathbf {F} _{\mathrm {fric} }$ is the friction force,

$\mathbf {F} _{\mathrm {n} }$ is the vector obtained by multiplying the magnitude of the normal force by a unit vector pointing against the object's motion,

$\mu _{\mathrm {k} }$ is the coefficient of kinetic friction, which is inside the integral because it may vary from location to location (e.g. if the material changes along the path),

$\mathbf {x}$ is the position of the object.

inherently rely on friction, slowing a vehicle by converting its kinetic energy into heat. Incidentally, dispersing this large amount of heat safely is one technical challenge in designing brake systems. Disk brakes rely on friction between a disc and brake pads that are squeezed transversely against the rotating disc. In drum brakes, brake shoes or pads are pressed outwards against a rotating cylinder (brake drum) to create friction. Since braking discs can be more efficiently cooled than drums, disc brakes have better stopping performance.^[79]

Automobile brakes

refers to the grip wheels of a train have on the rails, see Frictional contact mechanics.

Rail adhesion

Road slipperiness

[80]

. Encyclopædia Britannica. Vol. 11 (11th ed.). 1911.

"Friction"

– tables of coefficients, plus many links

Coefficients of Friction

Measurement of friction power

from the University of New South Wales

Physclips: Mechanics with animations and video clips

– CRC Handbook of Chemistry and Physics

Values for Coefficient of Friction

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