** Next:** Experimental facts about friction
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- 1.
- The
**Static force of friction**(*f*_{s}) is the force of friction between two objects when there is no motion. - 2.
- The
**Kinetic force of friction**(*f*_{k}) is the force of friction between two objects when there is motion.

- if
*F*_{ext}<*f*_{s(max)}the block won't move - as
*F*_{ext}increases,*f*_{s}will increase until it reaches its maximum value. When*F*_{ext}=*f*_{s(max)}the block will start to move (this is called the**point of slipping**). - Once the block starts to move, the force of friction is given by
*f*_{k}.

10/9/1997