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<HTML><HEAD><TITLE>new</TITLE><META content="text/html; charset=gb2312" http-equiv=Content-Type><LINK href="text.css" rel=stylesheet type=text/css><META content="Microsoft FrontPage 4.0" name=GENERATOR></HEAD><body leftmargin="15"><center><b><br>前言</b></center><table border="0" cellpadding="0" cellspacing="0" width="560"> <tr> <td> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr> <td width="50%" colspan="2"><b>例A：考虑解题步骤</b></td> <td width="50%" rowspan="5"> <p align="center"><img border="0" src="pic2/3070.h49.gif" width="201" height="206"></td> </tr> <tr> <td width="25%">如图，系统平衡。已知Q、l、α，求P。</td> <td width="25%" rowspan="4"><img border="0" src="pic2/3070.h50.gif" width="178" height="122"></td> </tr> <tr> <td width="25%">分析：</td> </tr> <tr> <td width="25%">1. 欲求P，可通过图(c)求，但NC、SBE未知；</td> </tr> <tr> <td width="25%">2. NC可通过整体求，图(a)；</td> </tr> </table> </td> </tr> <tr> <td>3. SBE可通过AEB求，图(b)。</td> </tr> <tr> <td> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr> <td width="33%"><b>解：</b></td> <td width="33%" rowspan="7"> <p align="center"><img border="0" src="pic2/3070.h51.gif" width="178" height="233"></td> <td width="34%" rowspan="7"> <p align="center"><img border="0" src="pic2/3070.h52.gif" width="180" height="167"></td> </tr> <tr> <td width="33%"> <table border="0" cellpadding="0" cellspacing="0"> <tr> <td><font color="#FF00FF">1.整体：</font> </td> <td><img border="0" src="pic2/3070.h53.gif" width="101" height="33"></td> </tr> <tr> <td></td> <td><img border="0" src="pic2/3070.h54.gif" width="62" height="33"></td> </tr> </table> </td> </tr> <tr> <td width="33%"><font color="#FF00FF">2. E点（或BE、AE及重物）</font></td> </tr> <tr> <td width="33%"><img border="0" src="pic2/3070.h55.gif" width="126" height="26"></td> </tr> <tr> <td width="33%"><img border="0" src="pic2/3070.h56.gif" width="66" height="29"></td> </tr> <tr> <td width="33%"><font color="#FF00FF">3. BC和滑块C</font></td> </tr> <tr> <td width="33%"></td> </tr> </table> </td> </tr> <tr> <td><img border="0" src="pic2/3070.h57.gif" width="101" height="31"> <img border="0" src="pic2/3070.h58.gif" width="52" height="27"></td> </tr> <tr> <td> <table border="1" cellpadding="0" cellspacing="0" bordercolor="#800080"> <tr> <td>结论：取3个分离体，列4个方程 ——较繁，尚可忍受！</td> </tr> </table> </td> </tr> <tr> <td> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr> <td width="50%"><b>例B：考虑解题步骤</b></td> <td width="50%" rowspan="8"> <p align="center"><img border="0" src="pic2/3070.h59.gif" width="183" height="307"></td> </tr> <tr> <td width="50%">如图，系统平衡。已知Q、l、 ，求P。</td> </tr> <tr> <td width="50%"> <table border="1" cellpadding="0" cellspacing="0" bordercolor="#800080"> <tr> <td>分离体太多！中间未知量太多！方程太多！<br> ——太繁！不能忍受 ！！！</td> </tr> </table> </td> </tr> <tr> <td width="50%"><b>怎么办？分析问题特点，引入新的求解思想：</b></td> </tr> <tr> <td width="50%"><u>结构特点</u>：几何可变体系。<br> <u>待求量特点</u>：较少，且具有主动力的性质。<br> <u>拓展思路</u>：可否直接建立P和Q 的关系？可否避开求中间反力？可否从动力学方程考虑？</td> </tr> <tr> <td width="50%"> </td> </tr> <tr> <td width="50%"><font color="#FF00FF">动量定理 或<br> 质心运动定理</font></td> </tr> <tr> <td width="50%"> <table border="0" cellpadding="0" cellspacing="0"> <tr> <td><img border="0" src="pic2/3070.h60.gif" width="93" height="51"></td> <td> 或 </td> <td><img border="0" src="pic2/3070.h61.gif" width="105" height="32"></td> </tr> </table> </td> </tr> </table> </td> </tr> <tr> <td> <table border="0" cellpadding="0" cellspacing="0"> <tr> <td></td> <td colspan="2"> <p align="center"><img border="0" src="pic2/3070.h62.gif" width="129" height="33"></td> <td rowspan="2"><img border="0" src="pic2/3070.h63.gif" width="13" height="82"></td> <td rowspan="2"> 还是静力学方程，无意义</td> </tr> <tr> <td><font color="#008000">动量矩定理：</font> </td> <td><img border="0" src="pic2/3070.h64.gif" width="120" height="52"></td> <td><img border="0" src="pic2/3070.h65.gif" width="139" height="39"></td> </tr> </table> </td> </tr> <tr> <td><font color="#800000">达朗伯原理？</font>仍然会得到纯静力学方程，也无效！</td> </tr> <tr> <td><font color="#0000FF">动能定理</font> 假设系统有一小的位移 ---→ <b>虚位移</b></td> </tr> <tr> <td> <table border="0" cellpadding="0" cellspacing="0" width="100%"> <tr> <td width="25%" align="center"><img border="0" src="pic2/3070.h66.gif" width="100" height="29"></td> <td width="21%" align="center"><img border="0" src="pic2/3070.h67.gif" width="96" height="27"></td> <td width="21%" align="center"><img border="0" src="pic2/3070.h68.gif" width="88" height="29"></td> <td width="33%" align="center">—虚功方程,即虚位移原理</td> </tr> <tr> <td width="25%" align="center"></td> <td width="21%" align="center"><img border="0" src="pic2/3070.h69.gif" width="55" height="21"></td> <td width="21%" align="center"> <p align="left">只包含P和Q,不含约束力,故建立P和Q的简单关系。</td> <td width="33%" align="center">↑用动力学思想<br> 解决静力学问题</td> </tr> </table> </td> </tr> <tr> <td>严格建立虚位移原理，需有诸多基本概念。</td> </tr> <tr> <td></td> </tr> <tr> <td> <p align="center"> <a href="3062.htm"><font color="#FF6666">[ 上一节 ]</font></a> <a href="3071_1.htm"><font color="#00CC00">[ 下一节 ]</font></a> </td> </tr></table></BODY></HTML> ⌨️ 快捷键说明
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