力学小知识请教
下面这个问题怎么用力学知识解释：同样一个简支梁，一个是变截面，一个等截面，A点到支点的距离相等，力矩相同，两种情况下A点的应力为啥不一样？谢谢！data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAABJcAAALZCAYAAAAKi8DOAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAABt8SURBVHhe7d2LbuNGEobR0b6in1PP6BUhM/Z4dOMvXrq6zwEK8YydTQZgKPFztff058+fz8sAAAAAwGL/+/orAAAAACwmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAGBo5/P56yMAABLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEADTnfD5/fdSXXv9cAMDYxCUAoDkfHx/dhZjpzzP9uQAAeiMuAQAAABATlwCAJvW0vWRrCQDombgEAAAAQExcAgCa1cP2kq0lAKB34hIA0LTKgUlYAgBGIC4BAAAAEBOXAIDmVdxesrUEAIxCXAIAAAAgJi4BACVU2l6ytQQAjERcAgDKqBCYhCUAYDTiEgAAAAAxcQkAKKXl7SVbSwDAiMQlAAAAAGLiEgBQTovbS7aWAIBRiUsAQEktBSZhCQAYmbgEAAAAQExcAgDKamF7ydYSADA6cQkAAACAmLgEAJR25PaSrSUAAHEJAOjAEYFJWAIAuBKXAAAAAIiJSwBAF/bcXrK1BADwTVwCAAAAICYuAQDd2GN7ydYSAMDfxCUAoCtbBiZhCQDgX+ISAAAAADFxCQDozhbbS7aWAABuE5cAAAAAiIlLAECX1txesrUEAHCfuAQAdGuNwCQsAQA8Ji4BAAAAEBOXAICuvbO9ZGsJAOA5cQkAAACAmLgEAHQv2V6ytQQA8BpxCQAYwpLAJCwBALxOXAIAAAAgJi4BAMN4ZXvJ1hIAwDLiEgAAAAAxcQkAGMqj7SVbSwAAy4lLAMBwbgUmYQkAICMuAQAAABA7Xebz+iEAwFh+bi/ZWgIAyNhcAgAAACBmc4n/3PvhpgAAAPTH1i5rEZf4jx9kCgAAMAbPf6zJsTgAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJS0BJ5/P55gAAALCv02U+rx8yuunB/OPj4+tX0KY5IN27Vp99HgAA8PzHumwuAWXML4CPXgTnz8+RCQAAgG2JS0AJS7+zIjABAADsQ1wCmpeu7ApMAAAA2xOXgKalYWkmMAEAAGxLXAIAAAAgJi4BzXp3a2lmewkAAGA74hIAAAAAMXEJAAAAgNjpMp/XDxmdY0MAAADjWONHUMBsiktm8Dmfz3/91ZgWZs3r0bVtjDHGGGPMdTz/mbXHsTgAAAAA3nKzOplx5netVq9NS7PG9eiaNsYYY4wx5jqe/8wWY3MJAAAABnA+n/2cJTYhLg3u1s1l+vX0+9CCd69HL6AAAHCf5z/WcnOlyfQ/lxvIzd+f59nnjdlzkuvRNWyMMcYYY8x1PP+ZLcfmElDC0u+oTF9rYwkAAGB7p8tMlYnBvPrg7QGd1syB6d51+ezzAAAwGs9/bE1cGpSbC9XNEek31ysAAHxb8kzn+Y+UuDSgpTcMNxgAAICaPP+xBz9zaTDJjWL6+unvAwAAoA7Pf+xFXAIAAIDO2EBiT+LSQN65uajXAAAAY/D8x1LiEgAAAHTE1hJ7E5cGscbNRb0GAABo21phyfMfS4hLA1izWrvBAAAAjMHzH68SlwAAAKADay4WwBLiUue2uLmo1wAAAGPw/McrxKWObVmt3WAAAADa4fmPI4lLAAAAUNiWYQleIS51ao+bi3oNAAAwBs9/PCIuAQAAQFG2lmiBuNShPW8u6jUAAMAx9g5Lnv+4R1zqzBHV2g0GAABgDJ7/uEVcAgAAgGKOWCyAe8Sljhx5c1GvAQAA9nF0WPL8x2/iEgAAAAAxcakTLaxEqtdU8vn5+fURAADU0cKz38TzHz+JSx1o5eYycYMBAAAYg+c/ZuISMJxpa+l0OtleAgCglJYWC+Ancam4Fm8u6jUAAMC6Wg1Lnv+YiEsAAAAAxE6XcS6kqAp12MomrZmPxM1+/xoAAFpTZTPI89+4xCVgKOISAADAuhyLAwAAACAmLgHDuLWlNP16+n0AAAAy4hIAAAAAMXEJAAAAgJi4BAzh0Q/udjQOAAAgJy4BAAAAEBOXAAAAAIiJS0D3Hh2JmzkaBwAAkBGXAAAAAIiJS0DXXtlaAgAAICcuAXxxNA4AAGA5cQkAAACAmLgEdMuROAAAgO2JSwA/OBoHAACwjLgEAAztfD5/fQQAQEJcArrkSBwAAMA+xCWAXxyNAwAAeJ24BAAAAEBMXAK640gcAADAfsQlgBscjQMAAHiNuAQAAABATFwCuuJIHAAAwL7EJYA7HI0DAAB4TlwCAAAAICYuAd1wJA4AAGB/4hIAAAAAMXEJ4AE/dwkAAOAxcQnogiNxAAAAxxCXAAAAAIiJSwBPOBoHAABwn7gElOdIHAAAwHHEJQAAAABi4hLACxyNAwAAuE1cAkpzJA4AAOBY4hIAAAAAMXEJKG/aXtpj5n8WAAAA38QloKwp9ExH4vYcAAAA/iYuAQAAABATl4CS5q2lvU3/TEfjAAAAvolLAAAAAMTEJQAAAABi4hJQzlFH4maOxgEAAHwTlwAAAACIiUsAAAAAxMQloJSjj8TNHI0DAAC4EpcAAAAAiIlLAAAAAMTEJaCMVo7EzRyNAwAAEJcAAAAAeIO4BAAAAEBMXAJKaO1I3MzROAAAYHTiEgAAAAAxcQkoocWtpVnL/24AAABbE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQBozvl8/vqoL73+uQCAsYlLAEBzPj4+ugsx059n+nMBAPRGXAIAAAAgJi4BAE3qaXvJ1hIA0DNxCQAAAICYuAQANKuH7SVbSwBA78QlAKBplQOTsAQAjEBcAgAAACAmLgEAzau4vWRrCQAYhbgEAAAAQExcAgBKqLS9ZGsJABiJuAQAlFEhMAlLAMBoxCUAAAAAYuISAFBKy9tLtpYAgBGJSwAAAADExCUAoJwWt5dsLQEAoxKXAICSWgpMwhIAMDJxCQAAAICYuAQAlNXC9pKtJQBgdOISAAAAADFxCQAo7cjtJVtLAADiEgDQgSMCk7AEAHAlLgEAAAAQE5cAgC7sub1kawkA4Ju4BAAAAEBMXAIAurHH9pKtJQCAv4lLAEBXtgxMwhIAwL/EJQAAAABi4hIA0J0ttpdsLQEA3CYuAQAAABATlwCALq25vWRrCQDgPnEJAOjWGoFJWAIAeExcAgAAACAmLgEAXXtne8nWEgDAc+ISAAAAADFxCQDoXrK9ZGsJAOA14hIAMIQlgUlYAgB4nbgEAAAAQExcAgCG8cr2kq0lAIBlxCUAAAAAYuISADCUR9tLtpYAAJYTlwCA4dwKTMISAEBGXAIAAAAgdrrM5/VDAICx/NxesrUEAJCxuQQAAABAzOYS/7n3w00BAADoj61d1iIu8R8/yBQAAGAMnv9Yk2NxAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BQ/r8/Pz6CAAAgHeISwAAAADExCVgONPW0ul0sr0EAACwAnEJAAAAgJi4BAAAAEBMXAKGMh+JmzgaBwAA8D5xCQAAAICYuAQAAABATFwChvHzSNzM0TgAAID3iEsAAAAAxMQlAAAAAGLiEjCEW0fiZo7GAQAA5MQlAAAAAGLiEgAAAAAxcQno3qMjcTNH4wAAADLiEgAAAAAxcQno2itbSwAAAOTEJYAvjsYBAAAsJy4BAAAAEBOXgG45EgcAALA9cQngB0fjAAAAlhGXAAAAAIiJS0CXHIkDAADYh7gE8IujcQAAAK8TlwAAAACIiUtAdxyJAwAA2I+4BHCDo3EAAACvEZcAAAAAiIlLQFcciQMAANiXuARwh6NxAAAAz4lLAAAAAMTEJaAbjsQBAADsT1wCAAAAICYuATzg5y4BAAA8Ji4BXXAkDgAA4BjiEgAAAAAxcQngCUfjAAAA7hOXgPIciQMAADiOuAQAAABATFwCeIGjcQAAALeJS0BpjsQBAAAcS1wCAAAAICYuAeVN20t7zPzPAgAA4Ju4BJQ1hZ7pSNyeAwAAwN/EJQAAAABi4hJQ0ry1tLfpn+loHAAAwDdxCQAAAICYuAQAAABATFwCyjnqSNzM0TgAAIBv4hIAAAAAMXEJAAAAgJi4BJRy9JG4maNxAAAAV+ISAAAAADFxCQAAAICYuASU0cqRuJmjcQAAAOISAAAAAG8QlwAAAACIiUtACa0diZs5GgcAAIxOXAIAAAAgJi4BJbS4tTRr+d8NAABga+ISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATl4CSzufzzQEAAGBf0/9/9uf1Q0Y3PZh/fHx8/QraNAeke9fqs88DAACe/1iXzSWgjPkF8NGL4Pz5OTIBAACwLXEJKGHpd1YEJgAAgH2IS0Dz0pVdgQkAAGB74hLQtDQszQQmAACAbYlLAAAAAMTEJaBZ724tzWwvAQAAbEdcAgAAACAmLgEAAAAQO13m8/oho3NsiMqeHZ9b64gdAMBv3kdTlffHrGmKS2bwubwg/vVXY1qYJdfjs691bRtjjDFmi/Eew1Sc+bp1/Zq1xrE4AAAAAN5yszqZceZ3rVavTUuz5Hq897WuaWOMMcZsMd5jmIrz+7p1HZs1xuYS0I3pzPjlxfHrVwAA25nec/h5NVTjumUr4tLgbt1cPKDTknevRy+gAABwn+c/1nJzpcn0P5cbyM3fn+fZ543Zc5Zcj/PXuoaNMcYYs8V4j2EqzrPr1nVt3hmbS0AJS76jMn+tjSUAYG3eYwD8S1wa1Csvikse5mEP8zX56Lp89nkAABjN9P7Y8x9bOl1mWmFiMK/cXCavfh3s7d4L38/r1fULAKzJewsqWnLdusZ5xz9n5Uzfc7lh3Pz9e7P0641paVy/xhhjjFljvKcwVcfzn9ljHIsbzOVGsbhET18//X0AAADU4fmPvYhLQNe8OAIA70oe0OForlv2JC4N5J2biwd0KnP9AgApD+iMyPtnlhKXAAAAoCOiKEe4+cOYTF9zubnc/P2ls9b/jjFHjOvXGGOMMUvGewdTcda8bv03YF4dm0sDuNwQVqvW1iOpzPULALxqzffQUJX3z7xKXAIAAIAOiKIcRVzq3BY3F/Wayly/AMAzHtDhm/fPvEJc6tiWL4puMAAA9EhYoirPfxxJXAKG48URAICeiKIcTVzq1B43Fw/oVOb6BQB+84AO93n/zCPiEgAAMDxhiapcu7RAXOrQnjcX9ZrKXL8AAFS2d1jy/pl7xKXO7H1zmbjBUJnrFwA44j00VOX9M7eISwAAwLCEJapy7dIScakjR95c1Gsqc/0CAFDJ0WHJ+2d+E5cALrxAAsB4jn5AB+iFuNSJFl4YPZwDAFCFsERVrVy7nv/4SVzqQEsvjG4wVOb6BQCA13n/zExcAvjBCyQA9K+lb87CEq5dWiUuFdfizcXDOQAArfJwTlWtXrue/5iISwC/eIEEAAB43ekyn9cPqabCw6/vClGZ72wCQF+8tlNVlW98+u9rXOISwB3egAJAP7yuA2zHsTiAO6Y3oFW+SwQAAHAUcQngAYEJAOqztQSwLXEJAADolrAEsD1xCeAJ20sAUJOwBLAPcQngBQITAADAbeISAADQHVtLAPsRlwBeZHsJAGoQlgD2JS4BLCAwAQAA/E1cAgAAumFrCWB/4hLAQraXAKBNwhLAMcQlgIDABAAAcCUuAQAA5dlaAjiOuAQQsr0EAG0QlgCOJS4BvEFgAgAARicuAQAAZdlaAjieuATwJttLAHAMYQmgDeISwAoEJgAAYFTiEgAAUI6tJYB2iEsAK7G9BAD7EJYA2iIuAaxIYAIAAEYjLgEAAGXYWgJoj7gEsDLbSwCwDWEJoE3iEgAAAAAxcQlgA7aXAGBdtpYA2iUuAWxEYAKAdQhLAG0TlwAAAACIiUsAG7K9BADvsbUE0D5xCWBjAhMAZIQlgBrEJQAAAABi4hLADmwvAcAytpYA6hCXAHYiMAHAa4QlgFrEJQAAAABi4hLAjmwvAcBjtpYA6hGXAHYmMAHAbcISQE3iEgAAAAAxcQngALaXAOBvtpYA6hKXAA4iMAHAlbAEUJu4BAAAAEDsdJnP64cAHMH2EgCjs7UEUJu4BAAAAEDMsTgAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAAxcQkAAACAmLgEAAAAQExcAgAAACAmLgEAAAAQE5cAAAAAiIlLAAAAAMTEJQAAAABi4hIAAAAAMXEJAAAAgJi4BAAAAEBMXAIAAAAgJi4BAAAAEBOXAAAAAIiJSwAAAADExCUAAAAAYuISAAAAADFxCQAAAICYuAQAAABATFwCAAAAICYuAQAAABATlwAAAACIiUsAAAAAxMQlAAAAAGLiEgAAAAChP3/+D9pEPjA6avKwAAAAAElFTkSuQmCC
简单地说，这两种情况 (等截面和变截面)，在 A 点的截面是不一样的，因而面积、抗弯惯矩等力学参数也不一样，当然应力也就不一样了。 图片如下，谢谢！ .....thanks!
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