Force resisting sliding motion / From Wikipedia, the free encyclopedia

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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.

Friction between two objects. The blue one has more friction against the sloped surface than the green one.
Figure 1: Simulated blocks with fractal rough surfaces, exhibiting static frictional interactions[1]

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.

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