问题 选择题

用长为 l 的细线,一端固定在 O 点,另一端系一质量为 m 的小球,小球可在竖直平面内做圆周运动,如图所示.MD 为竖直方向上的直径,OB 为水平半径,A 点位于 M、B 之间的圆弧上,C 点位于 B、D 之间的圆弧上,开始时,小球处于圆周的最低点 M,现给小球某一初速度,下述说法正确的是

A.若小球通过 A 点的速度大于,则小球必能通过 D 点

B.若小球通过B 点时,绳的拉力大于3mg,则小球必能通过 D 点

C.若小球通过 C 点的速度大于,则小球必能通过 D点

D.小球通过 D 点的速度可能会小于

答案

AB 

题目分析:当小球在D点时的向心力是只有重力充当时,小球恰好能通过D点,所以,即,此速度为临界速度,故D错误,若以此临界速度通过最高点,小球到M点时的速度为,解得,故A正确,到达B点时的速度为此时的拉力为,解得,此时绳子拉力为,所以若小球通过B 点时,绳的拉力大于3mg,则小球必能通过 D 点,B正确,因为C点距离B点的位置不明,如果要想小球能顺利通过D点,则通过C点的速度必须大于,C错误,

点评:对于圆周运动一定要弄清楚小球通过最高点的临界条件,然后根据能量守恒定律解题

阅读理解

任务型阅读。

     According to one study, words send only 7 percent of a person's message. Intonation (语调) and voice

quality communicate 38 percent, and nonverbal (not using or involving words) cues (暗示) transmit a large

55 percent. That means people pick up more from nonverbal communication than from the words a person

says. When studying a foreign culture then, it just makes sense to pay attention to how people use nonverbal

cues.

     Gestures consist of a major form of nonverbal communication. But often these gestures are culture-bound.

For example, when the Maoris of New Zealand stick out (伸出) their tongue at someone, it is a sign of respect.

When American schoolchildren make the same gesture, it means just the opposite. Also, Americans often

indicate "OK" with their thumb and fore-finger touching to form a circle. The same gesture means "money"

to the Japanese and "zero" to the French. For that reason, people in a foreign culture must use gestures with

caution.

     Another part of nonverbal communication is the one that you might not think about-space. When someone

comes too close, he feels uncomfortable. When he knocks into someone, he feels obligated (有义务的) to

apologize. But the size of a person's "comfort zone" varies, depending on his cultural or ethnic origin. For

example, in casual conversation, many Americans stand about four feet apart. People in Latin or Arabic cultures,

instead, stand very close to each other and touch each other often.

     Considering the effects of nonverbal communication, we never really stop communicating. How we walk,

how we stand and how we use our hands all send a message to others. That's why it's possible to "read

someone like a book".

1. How does an American feel when an Arab stands too close to him? (no more than 3 words)

    ____________________________________________________________________________________

2. What does the gesture "OK" mean in Japan? (no more than 3 words)

    ____________________________________________________________________________________

3. Why do we pay more attention to nonverbal cues in a foreign country? (no more than 10 words) 

    ____________________________________________________________________________________

4. Why should people in a foreign culture must use gestures with caution? ( no more than 10 words) 

    ____________________________________________________________________________________

填空题