Editing Friction (section)

===Static friction===<!-- Traction (engineering) links here -->
[[File:Static kinetic friction vs time.png|400px|right|thumb|When the mass is not moving, the object experiences static friction.  The friction increases as the applied force increases until the block moves.  After the block moves, it experiences kinetic friction, which is less than the maximum static friction.]]
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Static friction is friction between two or more solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as ''μ''<sub>s</sub>, is usually higher than the coefficient of kinetic friction. Static friction is considered to arise as the result of surface roughness features across multiple length scales at solid surfaces. These features, known as [[asperities]] are present down to nano-scale dimensions and result in true solid to solid contact existing only at a limited number of points accounting for only a fraction of the apparent or nominal contact area.<ref>[https://www.researchgate.net/publication/283675011_Static_friction_at_fractal_interfaces multi-scale origins of static friction] {{Webarchive|url=https://web.archive.org/web/20210918100737/https://www.researchgate.net/publication/283675011_Static_friction_at_fractal_interfaces |date=2021-09-18 }} 2016</ref> The linearity between applied load and true contact area, arising from asperity deformation, gives rise to the linearity between static frictional force and normal force, found for typical Amonton–Coulomb type friction.<ref>{{cite journal | author= Greenwood J.A. and JB Williamson| title= Contact of nominally flat surfaces | journal= Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences | volume=295 | issue=1442 | year=1966}}</ref>

The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: <math>F_\text{max} = \mu_\mathrm{s} F_\text{n}</math>. When there is no sliding occurring, the friction force can have any value from zero up to <math>F_\text{max}</math>. Any force smaller than <math>F_\text{max}</math> attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than <math>F_\text{max}</math> overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction. However, an apparent static friction can be observed even in the case when the true static friction is zero.<ref>{{Cite journal|last1=Nakano|first1=K.|last2=Popov|first2=V. L.|date=2020-12-10|title=Dynamic stiction without static friction: The role of friction vector rotation| url=https://link.aps.org/doi/10.1103/PhysRevE.102.063001| journal=Physical Review E|volume=102|issue=6|page=063001| doi=10.1103/PhysRevE.102.063001|pmid=33466084 |bibcode=2020PhRvE.102f3001N |hdl=10131/00013921 |s2cid=230599544 |hdl-access=free}}</ref>

An example of static friction is the force that prevents a car wheel from slipping as it rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, so it is static rather than kinetic friction. Upon slipping, the wheel friction changes to kinetic friction.  An [[anti-lock braking system]] operates on the principle of allowing a locked wheel to resume rotating so that the car maintains static friction.

The maximum value of static friction, when motion is impending, is sometimes referred to as '''limiting friction''',<ref name="Bhavikatti-1994">{{cite book
| url = https://books.google.com/books?id=4wkLl4NvmWAC&pg=PA112
| title = Engineering Mechanics
| last = Bhavikatti
| first = S. S.
| author2 = K. G. Rajashekarappa
| page = 112
| access-date = 2007-10-21
| publisher = New Age International
| isbn = 978-81-224-0617-7
| year = 1994
| archive-date = 2024-10-07
| archive-url = https://web.archive.org/web/20241007091508/https://books.google.com/books?id=4wkLl4NvmWAC&pg=PA112#v=onepage&q&f=false
| url-status = live
}}</ref>
although this term is not used universally.<ref name="Beer-1996"/>