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Static friction and kinetic friction

Friction forces are categorized as either static or kinetic. The coefficient of static friction ìs, characterizes friction when no movement exists between the two surfaces in question, and the kinetic coefficient ìk, characterizes friction where motion occurs. While static and kinetic friction differ in value (the coefficient of static friction typically being greater than that of kinetic friction), both result from the electric force acting on microscopic irregularities in two adjacent surfaces. It should be noted the friction force depends mostly on the resultant force acting on the body, while the motion of the body only influences the distinction between static or kinetic. While it is easier to visualize friction in terms of motion, its dependence is on the Resultant Force acting on a body.

The static friction force must be overcome by an applied force before an object will move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: Fmax = ìsN. It is important to realize that when there is no sliding occurring, the friction force can have any value from zero up to Fmax. Any force smaller than Fmax attempting to slide on surface over the other will be opposed by a frictional force of equal magnitude and opposite direction. Any force larger than Fmax, will overcome the force of static friction and cause sliding to occur. The instant that sliding occurs, kinetic friction is applicable, and static friction is no longer relevant.

When the Resultant Force acting along the plane of a surface increases in value, a proportional increase in Static Friction Force results, up to a maximum value Fmax. Since this is the maximum value which Static Friction can take for any particular material, a further increase in the Resultant Force will produce motion. A lower value of friction, kinetic friction, replaces static friction for the duration of the movement.

When one surface is sliding over the other, the friction force between them is always the same, and is given by the product of the coefficient of kinetic friction and the normal force: F = ìkN. The coefficient of static friction is larger than the coefficient of kinetic friction: it takes more force to make surfaces start sliding over each other than it does to keep them sliding once started.

These empirical relationships are only approximations: they do not hold exactly. For example, the friction between surfaces sliding over each other may depend to some extent on the contact area, or on the sliding velocity. The friction force is electromagnetic in origin: atoms of one surface "stick" to atoms of the other briefly before snapping apart, causing atomic vibrations, and thus transforming the work needed to maintain the sliding into heat. It should also be noticed that the coefficient changes slightly along a surface and that the coefficient is simply an approximation. However, despite the complexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in many applications.

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