Normally, I will add
[code]
cta = Math.atan2(x,y);
R = Math.sqrt(x*x+y*y);
[/code]
in the [b]drag[/b] action.

And add
[code]
x=R*Math.cos(cta);
y=R*Math.sin(cta);
[/code]
in the [b]fixed relation[/b] section.

Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=190.0]Circular motion in vertical plane[/url]
(It has been updated so that the particle is drag-able)