Forces of static friction and normal forces are similar in that they tend to prevent the surfaces of objects from moving with respect to one another.  For example, normal force interaction might prevent a book from falling through a table.  Static friction might prevent a bicycle tire from sliding on the pavement. Because these force interactions exist to prevent relative motion the forces will only be as big as they need to be and no bigger.

For example, when a cyclist is riding up a hill the force of static friction between the tires and the pavement will only be as large as is necessary to prevent the tires from slipping on the pavement.  The relationship:

\(| \overrightarrow{ f_{s} } |  \leq    \mu _{s}  | \overrightarrow{n} |\)

includes two important ideas.

1. The force of static friction will be any strength less than \(\mu _{s}  | \overrightarrow{n} |\) which is necessary to prevent the tires from slipping.
2. The force of static friction cannot be larger than \(\mu _{s}  | \overrightarrow{n} |\).  Before that could happen the tires would slip.