There is a formula that relates the arc length of a circle of radius, r, to the central angle ,$$ \theta$$ in radians.
Formula for $$ S = r \theta $$
The picture below illustrates the relationship between the radius, and the central angle in radians. The formula is $$ S = r \theta $$ where s represents the arc length, $$ S = r \theta$$ represents the central angle in radians and r is the length of the radius.
Demonstration of the Formual$$ S = r \theta$$
The interative demonstration below illustrates the relationship between the central angle of a circle, measured in radians, and the length of the intercepted arc. Just click and drag the points.
Practice Problem
Use the $$ S = r \theta $$ formula
S = 3(Π/3) = Π
Use the formula for S = r Θand calculate the intercepted arc: 6Π
Use the formula for S = r Θ and calculate the intercepted arc: 4Π
Use the formula for S = r Θ and calculate the solution

Further Reading:
 Unit Circle Game Free online game on all things about the unit circle
 Unit Circle Printables Images of blank unit circles and blank unit circles with the answers filled in
 Unit Circle Worksheet