private function rotateX(ball:animation_ch18_Ball3D, angleX:Number):void
    {
     var position:Array = [ball.xpos, ball.ypos, ball.zpos];
     
     var sin:Number = Math.sin(angleX);
     var cos:Number = Math.cos(angleX);
     var xRotMatrix:Array = new Array();
     xRotMatrix[0] = [1,    0,   0];
     xRotMatrix[1] = [0,  cos, sin];
     xRotMatrix[2] = [0, -sin, cos]
     
     var result:Array = matrixMultiply(position, xRotMatrix);
     ball.xpos = result[0];
     ball.ypos = result[1];
     ball.zpos = result[2];
    }