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When two surfaces are in contact, there is an electrical attraction between atoms of one surface and those of another. This is called friction. There are two kinds of friction:

  • Kinetic (sliding) friction is when one surface is moving across another.
  • Static friction is when the two surfaces are not moving across each other.

When you place a book on a table, there are two forces acting upon it: gravitational force and normal force. Normal force is perpindicular to the table, and cancels the weight of the book. This prevents the book from crashing through the table. Normal force is abbreviated FN or N (not to be confused with the abbreviation for a newton).