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In physics, friction is the resistive force, the physical deformation and the heat buildup that occurs when two surfaces travel along each other whilst forced together.

The friction-force is a function of the force pressing the surfaces together and the friction coefficient of the material interface. The initial force needed to overcome friction (static friction, or stiction, written μs) and start movement is greater than or equal to the force needed to sustain a movement, known as kinetic friction (μk). Since the total amount of friction depends on the path an object takes, friction is not a conservative force.

Physical deformation is a consequence of friction-force. Whereas this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and no longer hold the specified tolerances.

The energy used to overcome friction turns into heat. The work of movement will translate into deformation and heat that in the long run may affect the material's specification and the friction coefficient itself. Friction can cause solid materials to melt.

Friction may occur between solids, gases and fluids or any combination thereof. See aerodynamics and hydrodynamics. Also, friction occurs inside deforming objects.

A common way to reduce friction is by using a lubricant that is placed between the two surfaces, often radically lessening the percentage of the work that is turned into heat, which is the coefficient of friction. The science of friction and lubrication is called tribology.