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如图所示，为供儿童娱乐的滑梯的示意图，其中AB为斜面滑槽，与水平方向的夹角为θ=37°；长L的BC水平滑槽，与半径R=0.2m的

1
4
圆弧CD相切；ED为地面．已知儿童在滑槽上滑动时的动摩擦因数μ=0.5，在B点由斜面转到水平面的运动速率不变，A点离地面的竖直高度AE为H=2m．（取g=10m/s2，sin37°=0.6，cos37°=0.8）试求：

（1）儿童在斜面滑槽上滑下时的加速度大小？

（要求作出儿童在斜面上运动时的受力分析图）

（2）儿童从A处由静止开始滑到B处时的速度大小？（结果可用根号表示）

（3）为了使儿童在娱乐时不会从C处平抛滑出，水平滑槽BC的长度L至少为多少？

答案

（1）设儿童下滑的加速度大小为a，对儿童进行受力分析如图所示，则有：

    

  mgsin37°-μmgcos37°=ma1   

      解得：a1=2 m/s2．                                        

    因为H=2 m，圆弧CD的半径R=0.8 m，

    所以AB的长度L1=

H-R
sin370
=3m

（2）设儿童滑到B点的速率为vB，则：vB2=2aL1，

    （或依动能定理：mg(H-R)-umgcos370(L1sin370)=

1
2
mvB2-0）

     解得：vB=2

3
m/s

（3）设儿童在C点恰做平抛运动滑出时的速率为vC，则：mg=m

vC2
R

     f=umg=ma2

-2a2L≤vC2-vB2

（或用动能定理：-umgL=

1
2
mvc2-
1
2
mvB2）

   解得：L≥1 m．

答：（1）儿童在斜面滑槽上滑下时的加速度大小为2 m/s2；（2）儿童从A处由静止开始滑到B处时的速度大小为2

3
m/s；（3）为了使儿童在娱乐时不会从C处平抛滑出，水平滑槽BC的长度L至少为1m．

完形填空

第三节 完形填空（共20小题；每小题1. 5分，满分30分）

阅读下面短文，从短文后所给的四个选项(A、B、C和D)中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。

I have a love-hate relationship with 8 o’clock classes. I like to get   21   that early; most students don’t . They drag in late and look sleepy and  22     at that early hour.

One day I got really angry with it. I was  23   about the topic (history of Indians),but no one else seemed to be. I moved  24     the classroom. I told stories and tries   25   . I got theatrical(表演似的). It was useless. Students’blank looks didn’t seem to  26    . I was about to start a major topic when I   27  down at my watch. It was already past the scheduled end of class. So I just ended   28    .

Watching the students running out of the door, I  29    my books, notes and papers and stepped out into the hall. I was tired and a bit  30     . I had put everything I knew into that classroom   31    but it had seemed good for nothing. Then  32    I started across the hall, a student coming out of the back door of the classroom  33    me . It was Randy. I thought he wanted to ask  34   I didn’t have those test papers   35    or maybe he wanted   36   to hand in a late paper or …However , It was   37  of that. With his eyes sparking(闪亮). Randy said “This is the first time I’ve not    38  a college class to end. ”

The tiredness lifted. I had done  39    than I thought. I don't know if Randy will ever40    how much he did for me that day.

21. A. changed         B. started    C. caught       D. asked

22. A. dull   B. lazy  C. dirty        D. strange

23. A. sure   B. curious    C. excited    D. worried

24. A. into   B. from       C. forward   D. about

25. A. luck         B. humor    C. experiment     D. success

26. A. change      B. loose       C. appear     D. think

27. A. broke       B. put   C. took        D. glanced

28. A. carelessly         B. hurriedly        C. suddenly        D. naturally

29. A. got through      B. took away       C. picked up       D. went over

30. A. anxious    B. delight     C. sick  D. discouraged

31. A. lesson       B. discussion      C. performance   D. content

32. A. as        B. because    C. if    D. till

33. A. ran after    B. shouted at      C. got along with        D.caught up with

34. A. when        B. why        C. whether   D. who

35. A. taken        B. passed     C. graded     D. checked

36. A. excuse       B. information     C. promise   D. permission

37. A. none  B. nothing   C. all   D. anything

38. A. attended   B. asked      C. wanted    D. chosen

39. A. better        B. worse      C. more       D. less

40. A. care         B. understand      C. think       D. forget
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问答题 论述题

刘备为什么要种菜，关张是如何看待这件事的？
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