private function spring(partA:animation_ch12_Ball, partB:animation_ch12_Ball):void
    {
     var dx:Number = partB.x - partA.x;
     var dy:Number = partB.y - partA.y;
     var dist:Number = Math.sqrt(dx * dx + dy * dy);
     if(dist < minDist)
     {
      var ax:Number = dx * springAmount;
      var ay:Number = dy * springAmount;
      partA.vx += ax;
      partA.vy += ay;
      partB.vx -= ax;
      partB.vy -= ay;
     }
    }
   }
  }