"Friction" is the force of two surfaces In contact, or the force of a medium acting on a moving object (that is air on an aircraft). When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy.
Applicable units are those of quantitykind:Force
http://en.wikipedia.org/wiki/Friction
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http://dbpedia.org/resource/Friction
"Friction" is the force of two surfaces In contact, or the force of a medium acting on a moving object (that is air on an aircraft). When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy.