Editing Friction (section)

===Kinetic friction===
'''Kinetic friction''', also known as '''dynamic friction''' or '''sliding friction''', occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as ''μ''<sub>k</sub>, and is usually less than the coefficient of static friction for the same materials.<ref>{{cite book|title=Statics: Analysis and Design of Systems in Equilibrium |publisher=Wiley and Sons|year=2005 |isbn=978-0-471-37299-8|page=618 |quote=In general, for given contacting surfaces, ''μ''<sub>k</sub> < ''μ''<sub>s</sub> |author1=Sheppard, Sheri|author2=Tongue, Benson H.|author3=Anagnos, Thalia|author1-link=Sheri D. Sheppard}}
</ref><ref>
{{cite book
| title = Engineering Mechanics: Statics
|author1=Meriam, James L. |author2=Kraige, L. Glenn |author3=Palm, William John | publisher = Wiley and Sons
| year = 2002
| page= 330
| quote = Kinetic friction force is usually somewhat less than the maximum static friction force.
| isbn = 978-0-471-40646-4}}</ref> However, [[Richard Feynman]] comments that "with dry metals it is very hard to show any difference."<ref>{{cite web
| url = http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
| title = The Feynman Lectures on Physics, Vol. I, p. 12–5
| publisher = Addison-Wesley
| year = 1964
| author1 = Feynman, Richard P.
| author2 = Leighton, Robert B.
| author3 = Sands, Matthew
| access-date = 2009-10-16
| archive-date = 2021-03-10
| archive-url = https://web.archive.org/web/20210310234726/http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
| url-status = live
}}</ref>
The friction force between two surfaces after sliding begins is the product of the coefficient of kinetic friction and the normal force: <math>F_{k} = \mu_\mathrm{k} F_{n}</math>. This is responsible for the [[Coulomb damping]] of an [[Oscillation#Damped oscillations|oscillating]] or [[Vibration#Types|vibrating]] system.

New models are beginning to show how kinetic friction can be greater than static friction.<ref name="Volokitin, A. I-2002" >{{cite journal |title=Theory of rubber friction: Nonstationary sliding |journal=Physical Review B |volume=65 |page=134106 |doi=10.1103/PhysRevB.65.134106 |author1=Persson, B. N. |author2=Volokitin, A. I |year=2002 |bibcode=2002PhRvB..65m4106P |issue=13 |url=http://juser.fz-juelich.de/record/25870/files/17249.pdf |access-date=2019-01-31 |archive-date=2021-09-18 |archive-url=https://web.archive.org/web/20210918100743/https://juser.fz-juelich.de/record/25870/files/17249.pdf |url-status=live }}</ref> In many other cases roughness effects are dominant, for example in rubber to road friction.<ref name="Volokitin, A. I-2002" /> Surface roughness and contact area affect kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.<ref>{{cite book |last=Persson |first=B. N. J. |title=Sliding friction: physical principles and applications |url=https://books.google.com/books?id=1jb-nZMnRGYC&q=kinetic+friction |access-date=2016-01-23 |year=2000 |publisher=Springer |isbn=978-3-540-67192-3 |archive-date=2024-10-07 |archive-url=https://web.archive.org/web/20241007091447/https://books.google.com/books?id=1jb-nZMnRGYC&q=kinetic+friction#v=snippet&q=kinetic%20friction&f=false |url-status=live }}</ref>

The origin of kinetic friction at nanoscale can be rationalized by an energy model.<ref>{{cite journal |last1=Makkonen |first1=L |year=2012 |title=A thermodynamic model of sliding friction |doi=10.1063/1.3699027 |journal=AIP Advances |volume=2 |issue= 1|page=012179 |bibcode=2012AIPA....2a2179M |doi-access=free }}</ref> During sliding, a new surface forms at the back of a sliding true contact, and existing surface disappears at the front of it. Since all surfaces involve the thermodynamic surface energy, work must be spent in creating the new surface, and energy is released as heat in removing the surface. Thus, a force is required to move the back of the contact, and frictional heat is released at the front.

[[File:Free body.svg|thumb|Angle of friction, ''θ'', when block just starts to slide]]