The resistance to lateral motion when one attempts to slide the surface of one object over another surface is called friction or traction. The force of friction depends on the normal force, which is the force pressing the two surfaces together. It also depends on the types of materials from which the surfaces are formed--some materials are more slippery than others. The coefficient of friction is a measure of the slipperiness between two surfaces: the larger the coefficient of friction, the less slippery the surfaces. For example, pulling a heavy wooden block (large normal force) across the floor requires more force than does pulling a light one (small normal force); and pulling the wooden block along on a surface of rubber (large coefficient of friction) requires more force than pulling the same block along on a surface of ice (small coefficient of friction).

Static friction and kinetic friction

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For a given pair of surfaces, there are two types of friction coefficient. The coefficient of static friction, $\backslash mu\_s$, applies when the surfaces are at rest with respect to one another, while the coefficient of kinetic friction, $\backslash mu\_k$, applies when one surface is sliding across the other.

The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: $F\_\{max\}\; =\; \backslash mu\_s\; N$. It is important to realize that when sliding is not occurring, the friction force can have any value from zero up to $F\_\{max\}$. Any force smaller than $F\_\{max\}$ attempting to slide one surface over the other will be opposed by a frictional force of equal magnitude and opposite direction. Any force larger than $F\_\{max\}$ will overcome friction and cause sliding to occur.

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: $F\; =\; \backslash mu\_k\; N$. 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. However, despite the complexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in many applications.

The normal force

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If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force $N$ between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g. If the object is on a tilted surface such as an inclined plane, the normal force is less because less of the force of gravity is perpendicular to the face of the plane. Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram. Depending on the situation, the calculation of the normal force may include forces other than gravity.

See also

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• Frictional force
• Angle of repose
• Drag (physics)

Force

Coeficiente de rozamiento | Wrijvingscoëfficiënt | Coeficiente de atrito | Kitkakerroin