（10分）如图所示，一半径为R的光滑圆环，竖直放在水平向右的的匀强电场中，匀强电场的电场强度大小为E。环上a、c是竖直直径的两端，b、d是水平直径的两端，质量为m的带电小球套在圆环上，并可沿环无摩擦滑动，已知小球自a点由静止释放，沿abc运动到d点时速度恰好为零，由此可知小球所受重力 （选填“大于”、“小于”或“等于”）带电小球所受的静电力。小球在 （选填“a”、“b”、“c”或“d”）点时的电势能最小。
等于 b
题目分析：根据动能定理，合力做的功等于动能的增加量；从a到d过程，有：
解得 
,即电场力与重力大小相等，,根据功能关系，由a运动b电场力向左，电场力做正功，电势能减小，故运动到b点时电场力做的功最多，,所以电势能减小的最多，即小球在b点时的电势能最小．
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| Most adults（成年人） once studied at school,had classes and did their homework every day. The same 1 is going on at school now. 2 it seems that doing weekend homework is 3 problem for the modern students. All the students should agree that weekend homework should be abolished（取消）.It’s 4 for them studying at school five days a week.They have a lot of interests.With homework to do on Saturday and Sunday,how can they find 5 to help around the house,go and watch a football or basketball game or a good film,join in family recreations（娱乐）,or just have 6 at home?Because of these or other activities,the homework can’t be finished until 7 .So their weekend homework is usually done in such a hurry that on Monday teachers are 8 and often threaten（威胁） to fail whole class of students because they know nothing about the 9 .If there were no weekend homework against the students’ wishes,they would be happy to go to school on Monday 10 having a good rest and learn what the teachers teach. | |||
| ( )1.A.thing ( )2.A.Also ( )3.A.no ( )4.A.not enough ( )5.A.friends ( )6.A.a rest ( )7.A.Monday afternoon ( )8.A.pleased ( )9.A.lesson ( )10.A.until | B.school B.But B.another B.enough B.time B.an exam B.Saturday afternoon) B.sorry B.games B.when | C.class C.Still C.one C.no good C.places C.a lesson C.Friday night C.unhappy C.interests C.before | D.homework D.Though D.other D.no use D.money D.a picnic D.Sunday night D.not worried D.activities D.after |