The resistance to relative motion that exists between two different substances in contact, be they two solids, a solid and a liquid, a solid and a gas, a liquid and a gas, etc. The difficulty in sliding a heavy box along a flat surface is one example of friction, while the force of a doctor blade against a gravure cylinder would be another. Friction is the force responsible for abrasion and the damage caused thereby.

Static friction is defined as the resistance of an object to the initiation of sliding, while kinetic friction is the force that resists an object's continued sliding once it has been initiated. The coefficient of friction is one means of mathematically determining the frictional forces that exist between two sliding objects.

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