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物理状态的变化总是从量的渐变到质的突变。所谓临界问题,就是指物体从一种运动形式(或性质、特征)转变为另一种运动形式(或性质、特征)时,物体所处的物理状态和物理条件。由于临界状态是发生突变的关键所在,在物理试题中又带有其隐蔽性,稍不留心,易导致错解。所以此类问题对考查学生思维的敏捷性和运用物理知识解决实际问题的能力,具有重要意义。在近几年来的高考试题中,屡有出现,应引起我们的重视。一、临界外力和临界加速度例1 如图1,光滑平面上迭放 A、B 两物体,m_A=4千克,m_B=5千克。对 A 施以 F_1为12牛的水平拉力时,AB恰能相对静止一起向右运动。若改为对 B 施以水平拉力 F_2,问当 F_2多大时 A、B 两物体开始相对滑动?分析此题中 A、B 两物体由相对静止到相
The change of physical state is always from the gradual change of quantity to the qualitative change. The so-called critical question refers to the physical state and physical condition of an object when it changes from one form of movement (or nature, feature) to another form of movement (or nature, feature). Because the critical state is the key to the occurrence of mutations, it also has its hidden nature in the physics questions. It is slightly unattended and easily leads to misunderstandings. Therefore, such questions are of great significance for examining the agility of students’ thinking and their ability to use physical knowledge to solve practical problems. In the high examination questions of recent years, there have been repeated appearances that should arouse our attention. A. Critical external force and critical acceleration Example 1 As shown in Fig. 1, two objects A and B are stacked on a smooth plane, m_A = 4 kg, and m_B = 5 kg. When A is given a horizontal pull of F 1 at 12 N, AB can move to the right with relative motion. If instead of applying a horizontal tension F2 to B, ask how much the two objects A and B will start to slide when F_2 is large. Analyze the two objects A and B from relatively stationary to phase.