问题 选择题
两根足够长的光滑导轨竖直放置,间距为L,底端接阻值为R的电阻.将质量为m的金属棒悬挂在一个固定的轻弹簧下端,金属棒和导轨接触良好,导轨所在平面与磁感应强度为B的匀强磁场垂直,如图所示.除电阻R外其余电阻不计.现将金属棒从弹簧原长位置由静止释放,弹簧的劲度系数为k,则(   )

A.释放瞬间金属棒的加速度小于重力加速度g

B.金属棒向下运动时,流过电阻R的电流先减小后增大

C.当金属棒运动的速度最大为v时,由题给条件,不能求出金属棒下落的高度

D.电阻R上产生的总热量小于金属棒重力势能的减少量

答案

答案:D

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Solving a problem can be broken down into several steps. First, the problem must be identified correctly. Psychologists refer (1) this step as problem representation. For many problems, figuring (2) which information is relevant and (3) is extraneous can be difficult and can interfere (4) arriving at a good solution. Clearly, before a problem can be solved, it must be obvious (5) the problem is, however, this is not as easy (6) it might seem. One obstacle to efficient problem representation is functional fixedness, that is, allowing preconceived notions and even prejudices (7) color the facts. Most people tend (8) see objects and events in certain fixed ways, and by being inflexible in viewing the problem, they may be unable to notice the tools (9) the solution. Once the problem is identified accurately, (10) , the second step consists (11) considering the alternatives for a solution. A common way to evaluate alternatives is to write them (12) and then make a list (13) advantages and disadvantages for each solution. Here again, people may be limited by prior experiences. Often people adopt mental sets (14) lead them to the same problem-solving strategies that were successful for problems (15) the past. Although that can be helpful most (16) the time, sometimes a new situation requires a different strategy. (17) that case, the mental set must be abandoned, (18) new alternatives must be explored.
After the alternatives have been compared, a strategy must be selected (19) among them. One way to avoid becoming mired in the options is (20) try the best option with a view to abandoning it for another (21) the results are unfavorable. This attitude allows many people to move on expeditiously (22) the next step—action. The strategy selected must be implemented and tested. If it solves the problem, no further action is necessary, but if (23) , then the cycle begins again, starting (24) problem identification. By continuing to review the problem (25) repeat the problem-solving steps, the solution can be improved upon and refined.