《第四章习题课动力学.ppt

第四章　牛顿运动定律 习题课　动力学中的临界问题、极值问题 第四章　牛顿运动定律 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 一、临界问题的处理方法 知识检索 1．概念：某种物理现象(或物理状态)刚好要发生或刚好不发生的转折状态，叫临界状态． 2．关键词语：在动力学问题中出现的“最大”、“最小”、“刚好”、“恰能”等词语，一般都暗示了临界状态的出现，隐含了相应的临界条件． 3．常见类型：动力学中的常见临界问题主要有三类：一是弹力发生突变时接触物体间的脱离与不脱离的问题；二是绳子的绷紧与松弛的问题；三是摩擦力发生突变的滑动与不滑动问题． Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 4．解题关键：解决此类问题的关键是对物体运动情况的正确分析，对临界状态的判断与分析，找出处于临界状态时存在的独特的物理关系，即临界条件． 常见的三类临界问题的临界条件 (1)相互接触的两个物体将要脱离的临界条件是：相互作用的弹力为零． (2)绳子松弛的临界条件是：绳的拉力为零． (3)存在静摩擦的系统，当系统外力大于最大静摩擦力时，物体间不一定有相对滑动，相对滑动与相对静止的临界条件是：静摩擦力达到最大值． Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 典题示例 如图所示，小球用细绳拴着静止在光滑的倾角为α＝37°的斜劈上，求下列两种情况下斜劈的加速度大小和方向． (1)小球恰好不压斜劈； (2)细绳的拉力恰好为零． 【思路点拨】　上述两种情况中的“恰好”状态是指小球不离开斜劈且与斜劈一起运动，两者加速度相等，故求解小球的加速度即可(转换研究对象)． 例1 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 尝试应用 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd. 3.如图，木箱内有一竖直放置的弹簧，弹簧上方有一物块；木箱静止时弹簧处于压缩状态且物块压在箱顶上．若在某一段时间内，物块对箱顶刚好无压力，则在此段时间内，木箱的运动状态可能为(　　) A．加速下降 B．加速上升 C．减速上升 D．减速下降 Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.