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A [[force]] resulting from the contact of two surfaces. This force opposes motion.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref>
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A [[force]] resulting from the contact of two surfaces. This force opposes motion.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref>. For solid bodies, one can distinguish two types of friction:
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;[[static friction]]
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:the force that is needed for motion to start if the surfaces are initially at rest with respect to each other.
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;[[dynamic friction]]
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:the force that is caused by friction once the surfaces are moving with respect to each other.
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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>
    
==References==
 
==References==
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