如图所示，光滑的长平行金属导轨宽度d＝50cm，导轨所在的平面与水平面夹角＝37°，导轨上端电阻R＝0.8，其他电阻不计，导轨放在竖直向上的匀强磁场中，磁感应强度B＝0．4T。金属棒ab从上端由静止开始下滑，金属棒ab的质量m＝0.1kg。(sin37°＝0.6，g＝10m/s2)
(1)求导体棒下滑的最大速度；
(2)求当速度达到5m/s时导体棒的加速度；
(3)若经过时间t，导体棒下滑的垂直距离为s，速度为v。若在同一时间内，电阻产生的热与一恒定电流I0在该电阻上产生的热相同，求恒定电流I0的表达式(各物理量全部用字母表示)。
18.75m/s 4.4m/s 
(1)当物体达到平衡时，导体棒有最大速度，有：
根据安培力公式有：
根据欧姆定律有：
解得：
(2)由牛顿第二定律有：
a=.4.4m/s
(3)根据能量守恒有：
解得：
These days it is found that school students hardly have any sports. Is it because they have no interests in sports? It may not be true. They often say they have 1 more important things to do. What are these important things? Exams! They have to 2 themselves ready for all kinds of exams and tests in school. So many of them almost become bookworms ( 书虫 ). In the past, they could do 3 they liked in summer holidays, but now they have to 4 all their time preparing for the exams. So 5 have kept them away from going in for sports. Because of the pressure from 6 their parents and teachers, they 7 to work harder and spend most of their time on books. As for the students themselves, they don't want to 8 the exams. So it is necessary to give up their school sports and give all their spare time to their studies. It's true that a good education cannot go 9 physical training and that if a quick mind hardly goes along with a strong body, you can 10 do anything well, and then how can you make great success in life?
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