Here is a site that clearly and thoroughly explains arc length and its relationship to radian measure of angles. There are example problems solved, problems for the student to attempt, and answers to the student problems. Point this site...

After demonstrating how to find arc length and sector area using radian measures, student can practice the concepts with the examples provided and play an interactive game involving angular velocity.

Here is a site that clearly and thoroughly explains the radian measure of angles. There are example problems solved, problems for the student to attempt, and answers to the student problems.

Concepts in Trigonometry are explained and illustrated here by a member of the math department at Borough of Manhattan Community College.