Editing Ice (section)

===Friction properties===
[[File:2011 Figure Skating WC Takahiko Kozuka.jpg|thumb|[[Takahiko Kozuka]] figure skating - an act which is only possible due to ice's low frictional properties]]
Ice is "[[slipperiness|slippery]]" because it has a low coefficient of friction. This subject was first scientifically investigated in the 19th century. The preferred explanation at the time was "[[Pressure melting point|pressure melting]]" -i.e. the blade of an ice skate, upon exerting pressure on the ice, would melt a thin layer, providing sufficient lubrication for the blade to glide across the ice.<ref name="Rosenberg-2005" /> Yet, 1939 research by Frank P. Bowden and T. P. Hughes found that skaters would experience a lot more friction than they actually do if it were the only explanation. Further, the optimum temperature for figure skating is {{convert|-5.5|C|F K|0}} and {{convert|−9|C|F K|0}} for hockey; yet, according to pressure melting theory, skating below {{convert|−4|C|F K|0}} would be outright impossible.<ref name="Chang2006" /> Instead, Bowden and Hughes argued that heating and melting of the ice layer is caused by friction. However, this theory does not sufficiently explain why ice is slippery when standing still even at below-zero temperatures.<ref name="Rosenberg-2005">{{cite journal |last1=Rosenberg |first1=Robert |title=Why Is Ice Slippery? |journal=Physics Today |date=2005 |volume=58 |issue=12 |pages=50–54 |doi=10.1063/1.2169444 |bibcode=2005PhT....58l..50R |doi-access=free }}</ref>

Subsequent research suggested that ice molecules at the interface cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semi-liquid state, providing lubrication regardless of pressure against the ice exerted by any object. However, the significance of this hypothesis is disputed by experiments showing a high [[coefficient of friction]] for ice using [[atomic force microscopy]].<ref name=Chang2006>{{cite news |first=Kenneth |last=Chang |title=Explaining Ice: The Answers Are Slippery |work=[[The New York Times]] |date=21 February 2006 |access-date=8 April 2009 |url=https://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all |url-status=live |archive-url=https://web.archive.org/web/20081211055112/http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all |archive-date=11 December 2008 }}</ref> Thus, the mechanism controlling the frictional properties of ice is still an active area of scientific study.<ref>{{cite journal |last1=Canale |first1=L. |title=Nanorheology of Interfacial Water during Ice Gliding |journal=[[Physical Review X]] |date=4 September 2019 |volume=9 |issue=4 |page=041025 |doi=10.1103/PhysRevX.9.041025 |arxiv=1907.01316 |bibcode=2019PhRvX...9d1025C |doi-access=free }}</ref> A comprehensive theory of ice friction must take into account all of the aforementioned mechanisms to estimate friction coefficient of ice against various materials as a function of temperature and sliding speed. 2014 research suggests that frictional heating is the most important process under most typical conditions.<ref name=Makkonen2014>{{cite journal|last1=Makkonen|first1=Lasse|last2=Tikanmäki|first2=Maria|title=Modeling the friction of ice|journal=Cold Regions Science and Technology|date=June 2014|volume=102|pages=84–93|doi=10.1016/j.coldregions.2014.03.002|bibcode=2014CRST..102...84M }}</ref>