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中学物理中有许多极值问题,有的极值出现在物理变化的开始状态,有的则是在物理变化的终了才出现极值。例如图1所示装置,电阻R与竖直放置的两平行金属导轨相接,金属杆ab可以沿金属导轨下滑,整个装置处在与框架平面垂直的匀强磁场中,ab由静止开始沿导轨无摩擦下滑,ab运动的最大加速度是在ab开始运动时,而ab的最大速度是在ab运动的最终状态。这一类问题学生解答起来没有太大的困难,也有一些问题极值是在物理变化的过程中间某一点出现。对于这一类问题有些学生常因为不仔细分析而错误地认为极值点在变化的
There are many extremum problems in the middle school physics, some extreme values appear in the beginning of the physical changes, while others only occur at the end of the physical changes. For example, as shown in FIG. 1 , the resistor R is connected with two parallel metal rails placed vertically, the metal rod ab can slide down the metal rail, the whole device is in a uniform magnetic field perpendicular to the plane of the frame, and the ab starts from rest along the guide rail. Without friction, the maximum acceleration of ab movement is when ab begins to move, and the maximum speed of ab is the final state of ab movement. Students in this type of problem do not have much difficulty in solving it, and there are some problems where extreme values occur at some point in the course of physical changes. For this type of problem, some students often mistakenly believe that extreme points are changing because they are not carefully analyzed.