Basic Features

Friction opposes relative motion between two two objects in contact. This force points along interface between the objects.

If there is relative motion --- kinetic friction.

If there is no relative motion --- static friction.

Question:  You're standing still and you then start to walk forward across a flat floor. Which friction force applies? Which direction does it point?

  1. Static friction, opposite to the direction you're walking.
  2. Static friction, in the direction you're walking.
  3. Kinetic friction, opposite to the direction you're walking.
  4. Kinetic friction, in the direction you're walking.











If you do not slip, the appropriate force is static friction. This force propels you forward and thus is in the direction of walking.