问题 计算题

如图所示,空间存在水平向右的匀强电场E和垂直纸面向外的匀强磁场B,一个质量为m、带电荷量为+q的小球套在不光滑的足够长的竖直绝缘杆上,从静止开始下滑。已知滑动摩擦系数为μ.

(1)求下滑过程中小球具有的最大加速度am,并求此时的速度v?

(2)求下滑过程中小球具有的最大速度vm?

答案

在第一阶段:,  …………2分

加速度最大时,     ……………………1分

其最大值am=g。            …………………………………1分

在第二阶段: ……………………2分

当a=0时,     ………………………………………………1分

速度最大:         ……………………1分

单项选择题
完形填空

三、完形填空:(共20小题;每小题1分,满分20分)

One day a mother rat and her babies were out in an open field. They were playing and having a good time when ___21___ a hungry cat came on the scene! It hid ___22___ a big tree and then ___23___ forward through tall grass ___24___ it could almost hear them talk. ___25___ the mother rat and her babies knew ___26___ had happened, the cat ___27___ from its hiding-place and started to run ___28___ them.

The mother rat and her babies all ___29___ at once. They hurried towards ___30___ home, which was under a pile of large stones. ___31___ the baby rats were ___32___ scared that they could not run very ___33___. Closer and closer the cat came. In no time the cat could catch ___34___. What was to be done?

The mother rat stopped running, ___35___ round and faced the cat, ___36___, “Wow! Wow! Wow-wow!” just like ___37___ angry dog. The cat was so surprised and ___38___ that it ran away.

The mother rat turned to her babies, “Now you see ___39___ important it is to learn ___40___ second language!”

21. A. naturally           B. suddenly           C. nearly             D. certainly

22. A. or                 B. between           C. by                D. behind

23. A. crawled            B. jumped            C. looked             D. climbed

24. A. before             B. when              C. until               D. while

25. A. Before             B. After              C. Unless             D. Otherwise

26. A. where              B. what              C. which             D. when

27. A. jumped            B. started             C. walked            D. appeared

28. A. over               B. through            C. after             D. against

29. A. stood              B. fled               C. left               D. cried

30. A. to                 B. for                C. its               D. their

31. A. Because            B. But               C. Therefore          D. Although

32. A. as                 B. much              C. so               D. very

33. A. freely              B. hardly             C. soon             D. quickly

34. A. ahead              B. down              C. that              D. them

35. A. turned            B. walked             C. jumped           D. ran

36. A. saying             B. speaking           C. shouting          D. telling

37. A. a                 B. an                 C. their             D. that

38. A. pleased            B. excited             C. frightened         D. worried

39. A. so                B. why                C. what             D. how

40. A. our               B. their                C. a                D. An