An air puck is attached to a string that passes through the center of an air table. A weight is attached to the end of the string. The ball is set into rotation in a horizontal circle. When the weight is released, creating a constant force inwards, the radius of the path is shortened and the puck speeds up.
A similar effect is seen when the string wraps around a center rod like a tether ball. This creates an increasing force, which causes the puck to inspiral more quickly.