i did it!

i assign another variable (x000,y000)
cta = -Math.atan2(y000,x000)+pi/2; // to make drag by angle
R = Math.sqrt(x000*x000+y000*y000); // to make radius vary as well
x= x000; // may not be needed
y= y000; // may not be needed
_initialize();

in fixed relation
x000=R*Math.sin(cta); // code for making object x000,y000 follow the equation of motion
y000=R*Math.cos(cta);


i reflect back in the method, i still don't see the codes that cause the jam in the first place, the difference is just x000,y000 assign another variables.
maybe the problem is
cta = Math.atan2(y,x);
R = Math.sqrt(x*x+y*y); in the old mass object (x,y).

while, it works now! enjoy! http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1892.msg6846#msg6846
thanks! this forum rocks! & i am working towards human kindís well-being too  ;D