[quote author=Fu-Kwun Hwang link=topic=1454.msg6832#msg6832 date=1278500749]
If the mass is in circular motion, then the tension $T=m\frac{v^2}{r}$.

What if we increase the tension so that $T>m\frac{v^2}{r}$.
Will the mass moving inward($\frac{dv_r}{dt}<0$) or moving outward($\frac{dv_r}{dt}>0$)?

Thank about it and then check with your equation!
[/quote]
I see I got the sign convention wrong!
I forgot the convention for r unit vector is positive outward.
Will change it to
T1 = m*v*v/r -(cst*cst/(m*r*r*r)-M*g) ;
thanks !!! :)

your masterful questioning allows to think and figure out the answer myself.

my remixed applet is here
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1883.0
enjoy!