Friction is the force that resists the relative motion of two solid surfaces or fluid layers that are in contact and sliding against each other. There are several types of friction including dry friction between solid surfaces, fluid friction between layers in a viscous fluid, lubricated friction where a fluid separates two surfaces, skin friction as a form of drag on solid bodies moving through fluids, and internal friction within deforming solid materials. When surfaces in contact move relative to each other, friction converts the kinetic energy of motion into heat. Friction can dramatically impact systems through wear degradation and damage over time and is an important consideration in the science of tribology.

1. Friction
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For other uses, see Friction (disambiguation).
Classical mechanics
History of classical mechanics
Timeline of classical mechanics
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Fundamental concepts[show]
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 Rigid body
 Rigid body dynamics
 Euler's equations (rigid body dynamics)
 Motion
 Linear motion
 New ton's law s of motion
 New ton's law of universal gravitation
 Euler's law s of motion
 Equations of motion
 Inertial frame of reference
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 Fictitious force
 Mechanics of planar particle motion
 Displacement (vector)
 Relative velocity

2.  Friction
 Simple harmonic motion
 Harmonic oscillator
 Vibration
 Damping
 Damping ratio
Rotational motion
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 Centrifugal force (rotating reference frame)
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3.  V
 T
 E
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements
sliding against each other. There are several types of friction:
 Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided
into static friction ("stiction") between non-moving surfaces, and kinetic friction between moving
surfaces.
 Fluid friction describes the friction between layers within a viscous fluid that are moving relative to
each other.[1][2]
 Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.[3][4][5]
 Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid.
 Internal friction is the force resisting motion between the elements making up a solid material while
it undergoes deformation.[2]
When surfaces in contact move relative to each other, the friction between the two surfaces
converts kinetic energy into heat. This property 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
heat whenever motion with friction occurs, for example when aviscous fluid is stirred. Another important
consequence of many types of friction can bewear, which may lead to performance degradation and/or
damage to components. Friction is a component of the science of tribology.
Friction is not itself a fundamental force but arises from fundamental electromagnetic forcesbetween the
charged particles constituting the two contacting surfaces. The complexity of these interactions makes the
calculation of friction from first principles impossible and necessitates the use of empirical methods for
analysis and the development of theory.