[Espy]

Nope, what's propelling the ball in a circle is inertia. And since the ball is swinging around at a constant speed, there's no acceleration in the "around" direction, and hence no force.

And, ick, that's some weird wording. How I understand it, the Cartesian coordinates (0, 1) [x=0, y=1] translates to (pi/2, 1) [theta=pi/2, r=1] in polar, and (0, -1) translates to (-pi/2, 1), so if x=0, theta is either -pi/2 or pi/2.