private function onEnterFrame(event:Event):void
    {
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch09_Ball = balls[i];
      move(ball);
      var dx:Number = ball.x - centerBall.x;
      var dy:Number = ball.y - centerBall.y;
      var dist:Number = Math.sqrt(dx * dx + dy * dy);
      var minDist:Number = ball.radius + centerBall.radius;