private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align=StageAlign.TOP_LEFT;
  
     ball = new animation_ch05_Ball();
     ball.x = stage.stageWidth / 2;
     ball.y = stage.stageHeight / 2;
     vx = Math.random() * 10 - 5;
     vy = -10;
     addChild(ball);
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }