“一听就懂，一做就错．”这是初中阶段同学们对物理学科的普遍反映．究其原因？多种多样！但不容忽略的一个因素是：同学们对常见的物理语言懵懵懂懂，不知其中隐含的“弦外之音”（已知条件），以致于做题时一筹莫展，一做即错．因此，在做题过程中，我们若能抓住这一关键，? “You can understand it as soon as you can. If you do it right, it is wrong.” This is a common reflection of students in the junior middle school at the physics discipline. The reason? Various! However, one factor that cannot be ignored is that students are ignorant of common physical languages, and they do not know the “external sound” (known conditions) implied by them, so that when they do their work, they can do nothing but do it wrong. Therefore, if we can grasp this key in the process of doing this, what?