Friction is the ability of touching surfaces to support tangential as well as normal loads. This page will only be concerned with dry friction, which as its name suggests occurs between dry surfaces: wet friction will be covered later.
When a block stands on a surface, the surface exerts a reaction force (R), which opposes the weight of the block and any applied force. The reaction can be resolved into a normal component (N), which acts perpendicular to the surface, and friction (F), which acts parallel to the surface and resists the motion of the block.
The angle between the reaction and the normal component is denoted by φ, in which case F=N tan φ. There is, however, an upper limit to the friction that can be exerted by the surface. When a stationary block is about to slide, φ becomes φS, the angle of static friction. Tan φS=μS, the coefficient of static friction. In this situation, therefore, the friction equation becomes FS=μS|N|. Note that only the magnitude of the normal reaction is needed.
In general, for a stationary block F≤μS|N|. The value of F can be found using equilibrium equations.
If the friction that would be needed to keep the block stationary is greater than FS, the block moves. The friction is then calculated using the equation F=μD|N|, where μD is the coefficient of dynamic friction (also known as sliding or kinetic friction). The values of μS and μD depend on the constituent materials of the touching surfaces.
Friction is useful in such things as fan belts and clutches, but should be minimised in such things as bearings.
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