如图所示，在无限长的光滑绝缘的水平直轨道上有两个带电小球a和b

问题选择题

如图所示，在无限长的光滑绝缘的水平直轨道上有两个带电小球a和b，a球品质为2m，带电量为＋q，b球品质为m，带电量＋2q，两球相距较远且相向运动，某时刻a、b球的速度大小依次为v和1.5v，由于静电斥力的作用，它们不会相碰，则下列叙述正确的是（）

A．两球相距最近时，速度大小相等、方向相同

B．a球与b球所受的静电斥力对两球始终做负功

C．a球一直沿原方向运动，b球要反向运动

D．a、b两球都要反向运动，但a球先反向

答案

题目分析：由于地面光滑，系统所受合外力为零，满足动量守恒条件，当两球速度相等，系统损失机械能最大，两球相距最近．水平方向系统动量守恒，由完全非弹性碰撞的知识可知，当两球速度相等时，两球相距最小，速度向右；A正确；由题意可知，a球动量大于b球动量，因此系统动量水平向右，故b球运动过程中将反向运动而a球将一直沿原方向运动，（或者根据牛顿第二定律分析，此时a、b速度大小一样，而b的减速的加速度大，故b先减为零，然后反向加速运动），因此静电斥力对b球先做负功后做正功，故BD错误，C正确．

故选AC．

点评：根据系统运动情况，选用正确规律求解是解题的关键，如在本题中，库仑力为内力，系统外力为零，因此动量守恒，利用动量守恒解答要简单很多．

完形填空。
I first went to hear a live rock concert when I was eight years old. My brother and his friends were all 1 of a heavy metal group called Black Wednesday. When they 2 that Black Wednesday were going to perform at our local theatre, they all bought 3 for performance. However, at the last minute, one of the friends couldn't go, so my brother 4 me the ticket.I was really 5 ! I remember the buzz (嘈杂声)of excitement inside the theatre as we all found our 6 . After a few minutes, the lights went down and everybody became 7 .I could barely make out the stage in the 8 . We waited. Then there was a roar from the crowd, like an explosion, as the first members of the band 9 the stage. My brother leaned over and shouted something in my ear, but I couldn't 10 what he was saying. The first song was already starting and the music was as 11 as a jet engine.I could 12 the drum beats and bass notes in my stomach. I can't recall any of the songs that the band played. I just 13 that I really enjoyed the show and didn't want it to 14 . But in the end, after three encores (加演), the show finished. We left the 15 and walked unsteadily out onto the pavement.I felt a little dizzy, as if I had just 16 from a long sleep. My ears were still 17 with the beat of the last song. After the 18 , I became a Black Wednesday fan too for a few years before getting into other kinds of music. Once in a while, 19 , I listen to one of their songs and 20 I'm back at that first show.
( )1. A. members ( )2. A. guessed ( )3. A. flowers ( )4. A. booked ( )5. A. relaxed ( )6. A. seats ( )7. A. comfortable ( )8. A. silence ( )9. A. fell upon ( )10. A. forget ( )11. A. loud ( )12. A. feel ( )13. A. realize ( )14. A. continue ( )15. A. party ( )16. A. escaped ( )17. A. aching ( )18. A. competition ( )19. A. though ( )20. A. decide	B. friends B. discovered B. drinks B. offered B. embarrassed B. entrance B. quiet B. noise B. got through B. hear B. heard B. touch B. understand B. delay B. theatre B. traveled B. burning B. performance B. otherwise B. regret	C. fans C. thought C. clothes C. returned C. excited C. spots C. senous C. darkness C. broke into C. repeat C. sweet C. enj oy C. believe C. finish C. opera C. benefited C. ringing C. interview C. instead C. conclude	D. volunteers D. predicted D. tickets D. found D. encouraged D. space D. nervous D. smoke D. stepped onto D. bear D. fast D. digest D. remember D. change D. stage D. woken D. rolling D. celebration D. besides D. imagine