Friction

Definition

Intensional definition

Friction is the component of contact force between two bodies with a surface of contact acting in a direction along (or parallel to) the surface of contact, or the tangent plane through the point of contact.

Tendency-based definition

Friction refers to a force exerted by two bodies on each other, through a surface of contact, that tends to oppose slipping of the two surfaces of contact against each other. Note that static friction opposes the tendency indicated by the direction of net external force along the plane of contact, while kinetic friction opposes the direction of actual motion.