如图所示，光滑绝缘的水平桌面上有A、B两个带电小球，A球固定不

问题选择题

如图所示，光滑绝缘的水平桌面上有A、B两个带电小球，A球固定不动，给B球一个垂直AB连线方向的初速度 vo，有关B球在水平桌面上运动，不可能发生的情况是

（A）速度、加速度都变小（B）速度、加速度都变大

（C）速度变小、加速度变大（D）速度的大小和加速度的大小都不变

答案

答案：C

分析：分AB为同种电荷和异种电荷两种情况来讨论，当为同种电荷时，B球要远离A球，当为异种电荷的时候，根据库仑力和向心力的大小关系来分别讨论可能的运动情况．

解答：解：若A、B为同种电荷，AB之间的库仑力为排斥力，并且力的方向和速度的方向不再一条直线上，所以B球一定做曲线运动，由于AB之间的距离越来越大，它们之间的库仑力也就越来越小，所以B球的加速度在减小，库仑力做正功，速度增大

若A、B为异种电荷，AB之间的库仑力为吸引力，当AB之间的库仑力恰好等于向心力的时候，B球就绕着A球做匀速圆周运动，此时B球速度的大小和加速度的大小都不变，故D正确

当AB之间的库仑力大于需要的向心力的时候，B球做向心运动，速度和加速度都要变大，故B正确

当AB之间的库仑力小于需要的向心力的时候，B球做离心运动，速度和加速度都要减小，故A正确．

所以只有C不可能，故选C．

点评：当AB为异种电荷的时候，B球可能做向心运动也可能做离心运动，当库仑力恰好等于向心力的时候，B球就绕着A球做匀速圆周运动．

完形填空。
American people have the habit of saying "Thank you" when something kind is done for them or something polite is said to them, no matter how easy the thing is. This habit is learned by people of many __1__ countries. You should say "Thank you" when someone __2__ you the salt on the table, when someone steps aside to let you __3__ in a corridor (走廊), when someone __4__ ahead of you keeps the door open __5__ you, when someone says your work is well __6__ or you have bought a nice thing, or your city is very beautiful. "Thank you"is __7__ not only between strangers or new friends, but also between old friends, parents and children, brothers and sisters, husbands and wives. Parents often tell __8__ children that they should say "Thank you" in answer to a kind word or act when they are only four __9__ five years old. Perhaps that's __10__ almost everyone has got this habit.
( ) 1. A. others ( ) 2. A. puts ( ) 3. A. pass ( ) 4. A. jumps ( ) 5. A. to ( ) 6. A. made ( ) 7. A. tell ( ) 8. A. his ( ) 9. A. but ( ) 10. A. why	B. another B. passes B. jump B. walks B. with B. done B. said B. theirs B. and B. because	C. other C. carries C. sing C. sits C. for C. built C. to say C. their's C. or C. when	D. else D. borrows D. dance D. walking D. on D. doing D. told D. their D. so D. reason