• 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

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Many people think of friction as a motion-destroying force. That is just so wrong.

Friction does not oppose motion. Friction opposes relative motion. As such, friction is as capable of creating motion as destroying it.  See video below for proof.

• 04 Forces