ActionScript 3.0 :: Ball Collision Inside Another Ball Using Gravity?

ball collision inside another ball using gravity.I have one large circle in the middle of the stage (its radius is 150 pixels). I have another circle inside of that one (20 pixels radius), this is the smaller ball that I would like to have bounce around inside of the larger one with gravity. When the small ball hits the edge of the larger circle, I would like it bounce inwards to the center of the larger circle. It keeps doing this until it eventually stops in the center of the larger circle at the bottom.