问题 选择题

如图所示,轻杆BO一端装在铰链上,铰链固定在竖直墙上,另一端装一轻滑轮,重为G的物体用细绳经滑轮系于墙上A点,系统处于平衡状态,若将A点沿竖直墙向上缓慢移动少许,设法使系统重新平衡,则细绳所受拉力F1和轻杆所受压力F2大小变化情况是

A.F1变小

B.F1不变

C.F2变小

D.F2不变

答案

BC

题目分析:以O为研究对象进行受力分析,根据平衡关系画出几何图形,根据各力的性质及变化可由力的合成得出F的变化。由于AOG为同一根绳,所以绳的拉力大小相等所以,的大小不随夹角的变化而变化,所以A选项错误B选项正确;物体受力平衡,故向上的拉力等于重物的重力,故重物对O点的拉力为G;对O受力分析,如图所示,O点受向下的拉力,AO方向的拉力及BO方向的支持力F2;三力的合力为零,则作出AO与DO的拉力的合力,应与BO对结点O的支持力大小相等方向相反;因物体的重力不变,故细绳的拉力不变,两绳对O点的作用力不变,而A上移,使AO、OD的夹角变大,从而可知两力的合力减小,而BO的支持力F2一定与AO、OD的合力大小相等、方向相反,故BO的支持力F2一定变小,轻杆所受压力F2与支持力是相互作用了,故选项C正确,D选项错误。

不定项选择题
阅读理解

阅读理解。

     Juanito Estrella has been a housekeeping manager on the US-based large passenger ship Carnival Spirit for

18 months and feels he has found the suitable position in his career (职业). He has always wanted to travel.

"I guess I am a really restless spirit. I like traveling, so when the chance came, I jumped at it," he says.

     The chance came in the form of a newspaper advertisement for work on cruise ships (游船). At the time

Estrella was the housekeeping manager at a Melbourne hotel. He applied and, after two interviews, a medical

check and police clearance, the job was his.

     Estrella is responsible (负责的) for the cleanliness of the ship, making sure that 160 crew work properly.

"I enjoy it because there is no other work-you wake up each day in a different place and a different culture.

It's exciting when you go to the next country and you don't understand the language," he says.

     Estrella likes being busy and getting to know people from all over the world. The 1,000 crew come from

94 countries, and Estrella has learnt Spanish and a little Croatian.

     But there is a downside. "You cannot get really drunk...because you have safety responsibilities to yourself

and others," he says. "You don't really think about home. You start to think about home only when you get

closer to your vacation and wonder what you'll be doing."

     Life on the ship is anything but cruising. Estrella and his fellows work at least 10 hours a day, seven days

a week. He warns the job is not for everybody. "You have to love being busy and be prepared to work every

day-and to give up drinking too much alcohol." In his spare time, if the ship sails into a port, Estrella explores

it, otherwise he works out in the crew's gym, goes on the internet or calls home.

1. What do we know about Estrella?

A. He is very fond of traveling.

B. He doesn't drink wine now.

C. He cannot speak a foreign language.

D. He used to be a housekeeping manager.

2. The underlined word "downside" in paragraph 5 probably means _____.

A. disappointment

B. disadvantage

C. failure

D. loss

3. Which of the following is true?

A. Estrella doesn't often feel homesick because of his work.

B. The work on the ship is not suitable for a married person.

C. Estrella can find no other job except the one on the ship.

D. The people on the ship are from 94 countries.

4. In the last paragraph, the writer thinks that life on the ship is _____.

A. not a tiring journey at all

B. just an interesting voyage

C. far from a voyage for pleasure

D. more than a pleasant travel by sea