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问题 选择题

图中ABCD是一条长轨道，其中AB段是倾角为θ的斜面，CD段是水平的，BC是与AB和CD都相切的一小段圆弧，其长度可以略去不计．一质量为m的小滑块在A点从静止状态释放，沿轨道滑下，最后停在D点，A点和D点的位置如图所示，现用一沿轨道方向的力推滑块，使它缓缓地由D点推回到A点，设滑块与轨道间的动摩擦系数为μ，则推力对滑块做的功等于（　　）

A．mgh

B．2mgh

C．μmg（s+

h
sinθ
）

D．μmgs+μmgshcosθ

答案

物体由A点下落至D点，设克服摩擦力做功为WAD，由动能定理：mgh-WAD=0，

      即   WAD=mgh  ①

由于缓缓推，说明动能变化量为零，设克服摩擦力做功为WDA，由动能定理

当物体从D点被推回A点，WF-mgh-WDA=0   ②

根据W=FLcosα可得：

由A点下落至D，摩擦力做得功为WAD=-μmgcosθ×

h
sinθ
-μmgs    ③

从D←A的过程摩擦力做功为，WDA=-μmgcosθ×

h
sinθ
-μmgs   ④

③④联立得：WAD=WDA   ⑤

①②③联立得：WF=2mgh  故ACD错误B正确，

故选B．

单项选择题

When I was looking for a Christmas present for my daughter in a toy store, a nicely dressed little girl, with some money in her little hand, was looking at some beautiful dolls. When she saw a doll she liked, she would ask her father if she had enough money. He usually said yes.
At the same time, a boy, with old and small clothes, was looking at some video games. He, too, had money in his hand, but it looked no more than five dollars. Each time he picked up one of the video games and looked at his father, he shook his head.
The little girl had chosen her doll, a very beautiful one. However, she noticed the boy and his father. She saw the boy give up a video game with disappointment and walk to another corner of the store.
The little girl put her doll back to the shelf and ran over to the video game. After she talked to her father, she paid for the video game and whispered(耳语) to the shop assistant.
So the boy got the video game that he wanted for free—he was told it was a prize from the store. He smiled happily, although he felt it was so incredible. The girl saw all this happen. She smiled, too.
When I walked out of the store to my car, I heard the father ask his daughter why she had done that. I would never forget their short talk. "Daddy, didn’t Grandma want me to buy something that would make me happy"
He said, "Of course, she did."
"Well, I just did!" With that, the little girl started skipping towards their car happily.

The story happened in a ______.

A. school
B. toy store
C. cinema
D. computer room
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多项选择题

（）外国人不准出境。

A、属于刑事案件被告人的

B、人民法院通知有未了结的民事案件不能离境的

C、违反治安管理尚未处理公安机关认定需要追究的

D、负有债务的
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