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A forve '''Friction''' is a [[force]] resulting from the contact of two surfaces. This force opposes motion.<ref>Wile, Dr. Jay L. ''Exploring Creation With BiologyPhysical Science''. Apologia Educational Ministries, Inc. 19981999, 2000</ref> Unlike gravity and electromagnetism, friction is not a [[conservative force]]. For solid bodies, one can distinguish two types of friction: ;[[Static friction]]:the force that is needed for motion to start if the surfaces are initially at rest with respect to each other.;[[Kinetic friction]]:the force that is caused by friction once the surfaces are moving with respect to each other. For solid surfaces, both types of friction depend (to a good approximation) only on the normal force. In particular, the dynamic friction does not depend on the velocity.<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref>