问题 多选题

如图所示,固定在竖直平面内的光滑圆环的最高点有一个光滑的小孔,质量为m的小球套在圆环上.一根细线的下端系着小球,上端穿过小孔用手拉住.现拉动细线,使小球沿圆环缓慢上移.在移动过程中手对线的拉力F和轨道对小球的弹力N的大小变化情况是(  )

A.F不变

B.F减小

C.N不变

D.N减小

答案

小球沿圆环缓慢上移可看做匀速运动,对小球进行受力分析,小球受重力G,F,N,三个力.满足受力平衡.作出受力分析图如下

由图可知△OAB△GFA

即:

G
R
=
F
AB
=
N
R

当A点上移时,半径不变,AB长度减小,故F减小,N不变,故A错误,B正确,C正确,D错误;

故选BC.

阅读理解

阅读理解。

  "How are you?" is a nice question. It's a friendly way that people in the

U.S.A greet each other. But "How are you?" is also a very unusual question. It's a

question that often doesn't need an answer. The person who asks "How are you?"

hopes to hear the answer"Fine", even if the person's friend isn't fine.The reason

is that "How are you?" isn't really a question and "Fine"isn't really an answer.

They are simply other ways of saying"Hello"or "Hi".

  Sometimes, people also don't say exactly what they mean. For example,

when someone Asks "Do you agree?", the other person might think, "No, I

disagree. I think you're Wrong..."But it isn't very polite to disagree so strongly,

so the other person might say"I'm not sure." It's a nicer way to say that you don't

agree with someone.

  People also don't say exactly what they are thinking when they finish talking

with other people.For example, many talks over the phone finish when one

person say "I've to go now."Often,the person who wants to hang up gives and

excuse:"Someone's at the door.""Something is burning on the stove."The excuses

might be real,or not. Perhaps the person who wants to hang up simply doesn't

want to talk any more,but it isn't polite to say that. The excuse is more polite,and

it doesn't hurt the other person.

  Whether they are greeting each other, talking about an idea, of finishing a

talk, people don't say exactly that they are thinking. It's an important way that

people try to be nice to each other, and it's part of the game of language.

1. When a person in America asks"How are you?", he or she wants to hear

"        "

A. How are you?

B. Hello

C. I don't know.

D. Fine

2. When a person disagree with someone, it is polite to say"        "

A. You are wrong.I disagree.

B. I'm not sure.

C. I'm sure I disagree.

D. No, I disagree.

3. A polite way to finish talk is to say"       "

A. You have to go now.

B. I want to hang up.

C. I have to go now.

D. I don't want to talk any more.

4. When a person says"I've to go now. Someone's at the door.", the person may

be         .

A. giving an excuse

B. hurting someone's feeling

C. ill

D. asking you a question

5. The rule of the game of language is probably       .

A. "Always say what you mean"  

B. "Don't disagree with people."

C. "Never say exactly what you're thinking."    

D. "Be polite."

填空题