Friction is a force that opposes motion in a particular direction. Friction occurs when the surfaces of 2 materials come into contact with each other. Since no surface is completely smooth, there will always be a frictional force that opposes motion. The magnitude of this force is dependent upon the material of the 2 surfaces and the normal force. (Physics Encyclopedia, "Friction", 1998)
Normal Force The normal force is always perpendicular to the surface of motion. On a flat surface, the normal force (N) is equal to the weight, mg, of the object in motion. However, on an inclined plane, the normal force is equal to mgcos? In the physics of hockey, the surface of the ice pad is parallel to a level ground. (Physics Encyclopedia, "Friction", 1998) The Two Types of Friction There are two types of friction that can occur when an object moves along a surface: Static and Kinetic. Static friction prevents motion from occurring altogether and occurs when objects are stationary. Kinetic friction, on the other hand, impedes motion already in progress.
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