private function onMouseDown(event:MouseEvent):void
    {
     oldX = ball.x;
     oldY = ball.y;
     stage.addEventListener(MouseEvent.MOUSE_UP, onMouseUp);
     ball.startDrag();
     removeEventListener(Event.ENTER_FRAME, onEnterFrame);
     addEventListener(Event.ENTER_FRAME, trackVelocity);
    }
    
    private function onMouseUp(event:MouseEvent):void
    {
     stage.removeEventListener(MouseEvent.MOUSE_UP, onMouseUp);
     ball.stopDrag();
     removeEventListener(Event.ENTER_FRAME, trackVelocity);
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }