Radian (rad)

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.