private function onEnterFrame(event:Event):void
    {
     var elapsed:Number = getTimer() - time;
     time = getTimer();
     vy += gravity * elapsed / 1000;
     ball.x += vx * elapsed / 1000;
     ball.y += vy * elapsed / 1000;
     var left:Number = 0;
     var right:Number = stage.stageWidth;
     var top:Number = 0;
     var bottom:Number = stage.stageHeight;