Coefficient of friction
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The resistance to lateral motion when one attempts to slide the surface of one object over another surface is called friction or traction. Depending on the type of materials that are in contact, the force of this property of matter can vary significantly. For example pulling the surface of a wooden block along on a steel surface would require more force than pulling the same block along on an ice surface.
For a given pair of surfaces, the coefficient of friction, or μ, is a constant. The product of the coefficient of friction and the normal force between two free bodies gives the value of the frictional force present in a given situation. If an object were on a level surface, the normal force which it exerts on the surface and vice versa would be its mass, m, multiplied by the acceleration due to earth's gravity, g, or mg (which is also known as a Newton, N). If this object were on a non-level surface such as an inclined plane, the normal force would be less because less of the force would be perpendicular to the face of the plane, as some of the gravity would now have a parallel component relative to the plane itself. Therefore, the normal force, and ultimately the frictional force would have to be determined using vector addition, which can include forces other than gravity, including electrostatic, magnetic or kinetic. Once this parallel component of the gravitation force overcomes the product of the normal force and coefficent of friction an object will begin to slide.
The coefficient of friction can also be seen as the degree of slip resistance. A situation where the objects are stationary relative to each other would requre the use of the static coefficient of friction, but a situation in which one or more of the objects in contact had a velocity would call for the use of the kinetic coefficient of friction. Both of these constants are used as described above, but they are different values, and represent different properties. For the same pair of surfaces, the coefficient of kinetic friction is usually lower than the coefficient of static friction. Once an object starts to slip the force needed to keep is slipping is less than the force that was initially needed to get it to slip.