Kinetic friction
This article describes a force type.
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Definition
Intensional definition
Kinetic friction, or dynamic friction, is defined as the form of friction between two bodies via their surface of contact (i.e., the force acting parallel to the plane of contact) when the two surfaces of contact are slipping against each other.
Tendency-based definition
Kinetic friction is the form of friction that opposes the slipping against each other of two surfaces in contact. Its direction is opposite the direction of relative motion of the surfaces in contact. Note that it need not oppose the net external force between the surfaces in contact.