A force on objects or substances in contact with each other that resists motion of the objects or substances relative to each other.♦ Static friction arises between two objects that are not in motion with respect to each other, as for example between a cement block and a wooden floor. It increases to counterbalance forces that would move the objects, up to a certain maximum level of force, at which point the objects will begin moving. It is measured as the maximum force the bodies will sustain before motion occurs.♦ Kinetic friction arises between bodies that are in motion with respect to each other, as for example the force that works against sliding a cement block along a wooden floor. Between two hard surfaces, the kinetic friction is usually somewhat lower than the static friction, meaning that more force is required to set the objects in motion than to keep them in motion. See also drag.

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