问题 问答题

某人在5s内,用如图所示的滑轮组,将重为1280N的物体匀速提升1m.已知动滑轮的总重为320N.不计绳重和摩擦.求:

(1)人做功的功率;

(2)滑轮组的机械效率.

答案

(1)拉力F=

G+G
4
=
1280N+320N
4
=400N;

拉力移动的距离S=4h=4×1m=4m;

拉力做的总功W=FS=400N×4m=1600J;

人做功的功率P=

W
t
=
1600J
5s
=320W;

答:人做功的功率为320W.

(2)有用功W=Gh=1280N×1m=1280J;

机械效率η=

W有用
W
=
1280J
1600J
=80%.

答:滑轮组的机械效率为80%.

判断题
阅读理解

阅读理解

     By analyzing academic data from 10 states,representing the testing of more than 7.2 millions of youths,US researchers found that gender differences in math scores were extremely small among all ethnic

groups in grades 211.

     These findings are in contrast to earlier results from 1990 that indicate measurable differences favoring males in complex problemsolving,beginning in the high school years.

    Janet Hyde from University of Wisconsin and colleagues reported their study results in the journal

Science on Friday.

      Currently,women are largely absent from the highest levels of careers in mathematics,the physical

sciences,and engineering.In the United States,Ph.D.programs in engineering currently average only about

15 percent women,and similar statistics have led to stereotypes (成见) about girls and women lacking in

mathematical ability.

     Now,it is clear that the general population no longer shows a gender difference in mathematical skills,

according to this latest report.

     And this new data on academic performance in the United States shows that students are tested frequently on "recall" and "skill/concept" items,but rarely on "strategic thinking" (战略思维) or "extended thinking" concepts.

If standardized tests do not assess the sorts of reasoning that are crucial to careers in science and math,

the skills may not be taught,putting American students at a disadvantage to students in other countries

where testing and instruction focuses on more challenging content."This is a gap that should be fixed,"

Hyde said.

1. What is the best title of the passage?

A. A new study result about math.

B. No gender differences in math.

C. Girls are not good at math.

D. Girls do better in math than boys.

2. Earlier research results________.

A. are the same as that of this latest report

B. support gender differences in math

C. indicate girls prefer to solve complex problems

D. are the roots of many stereotypes about women

3. The underlined word "crucial" in the last paragraph most probably means________.

A. limited  

B. available  

C. important  

D. equal

4. What Hyde said in the last paragraph means that________.

A. standardized tests should assess different classes around the world

B. testing and instruction should focus on more challenging content in the USA

C. students in other countries are cleverer than American students

D. students should be tested frequently on "recall" and "skill/concept" items

5. The passage is most probably taken from________.

A. a learning guide  

B. a research paper

C. a personal diary  

D. a newspaper