Reply to lvleph at post # 16:
Take a closer look at post #3. Stability, in this context, is dealing with the boundedness of y. If y is bounded, you've got stability. Otherwise, you don't.
While we already are on the subject of physics. I could need some help understanding the derivatives of polar coordinates:
If I take the derivative with respect to time for theta(hat) - dont know how to type with math symbols
What will I get?
I know that if i do it for the radial unit vector i get - r(hat)' = theta'*theta(hat)
I can see it for the radial unit vector but not for the angular one. Any one dare to give an explanation?