https://blog.gongquiz.com/2021/02/22/%eb%aa%a8%ec%96%b4%ec%9d%91%eb%a0%a5%ec%9b%90mohr-stress-circle%ec%97%90%ec%84%9c-%ec%9d%91%eb%a0%a5%ec%9b%90%ec%9d%98-%eb%b0%98%ea%b2%bd%ec%9d%80-%eb%ac%b4%ec%97%87%ec%9d%84-%ec%9d%98%eb%af%b8/