private function rotateY(ball:animation_ch15_Ball3D, angleY:Number):void
    {
     var cosY:Number = Math.cos(angleY);
     var sinY:Number = Math.sin(angleY);
     
     var x1:Number = ball.xpos * cosY - ball.zpos * sinY;
     var z1:Number = ball.zpos * cosY + ball.xpos * sinY;
     
     ball.xpos = x1;
     ball.zpos = z1;
    }