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Friction is a force resulting from the contact of two surfaces. This force opposes motion.[1] 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.[2]


  1. Wile, Dr. Jay L. Exploring Creation With Physical Science. Apologia Educational Ministries, Inc. 1999, 2000
  2. Marcelo Alonso and Edward J. Finn, Fundamental University Physics, Addison-Wesley.