• Contact force arises when two surfaces come into contact. This contact force is often resolved into two components:
  • The component perpendicular to the surfaces is called the normal contact force N. This force arises because the two surfaces are pressing into each other.
  • The component parallel to the surfaces is called friction f. This force arises when two surfaces are sliding, or are tending to slide, relative to each other.

• The direction of friction is always to (try to) prevent the two surfaces to slide relative to each other.
• Sometimes friction retards motion.

• Sometimes friction produces motion

• The frictional force between two particular surfaces depends only on (1) the normal contact force N (relating to how hard the two surfaces are pressed together) and (2) the coefficient of friction μ (relating to how “sticky” the two surfaces are to each other).

• f = μN

(more detail)

• 03 Dynamics