In physics we sometimes measure angles (q) in radians (rad). To find an angle in radians we divide the arc length (s) by the radius (r).

Now lets consider a full circle. Once round a full circle is the circumference of the circle and we know that the circumference of a circle is given by 2pr so the angle in radians of a full circle is;

the r cancels to give;

The key thing for us to remember is that once round a full circle is 2p radians.