问题 选择题

质量为m的消防队员从一平台上自由落下,下落3m后双脚触地,接着他用双腿弯屈的方法缓冲,使自身重心又下降了0.6m,假设在着地过程中地面对他双脚的平均作用力大小恒定,则消防队员(  )

A.着地过程中地面对他双脚的平均作用力等于6 mg

B.着地过程中处于失重状态

C.在空中运动的加速度大于触地后重心下降过程中的加速度

D.在着地过程中地面对他双脚做负功使其动能减小

答案

A、设消防队员的重力为G,地面对他双脚的平均作用力大小为F.根据动能定理,对全过程进行研究得:G(h1+h2)-Fh2=0得到F=6G,即在着地过程中地面对他双脚的平均作用力估计为自身所受重力的6倍,A正确.

B、着地过程人做减速运动,加速度向上,处于超重状态,B错误;

C、由公式

V20
=2as知在空中运动的加速度小于触地后重心下降过程中的加速度,C错误;

D、在着地过程中地面对他双脚做功为零,因为着地后脚的位移为零,D错误;

故选A

阅读理解

Students who say they never or hardly ever used dictionaries may speak English well but usually write poorly, because they make many mistakes.

The students who use dictionaries most do not learn especially well either. The ones who look up every new word do not read fast. Therefore they do not have time to read much. Those who use small two-language dictionaries have the worst problems. Their dictionaries often give only one or two words as translations of English. But one English word often has many translations in a foreign language and one foreign word has many translations in English.

The most successful students are those who use large college edition dictionaries with about 100,000 words but do not use them too often. When they are reading, these students first try to get the general idea and understand new words from the context. Then they reread and use the dictionary to look up only key words that they still do not understand. They use dictionaries more for writing. If they are not sure how to spell a word, they always use a dictionary. Also, if they think a noun might have an unusual plural form, they check this in a dictionary.

小题1:The writer thinks that        .

A.choose a good dictionary, and you’ll be successful in learning English

B.dictionaries are not necessary to the students who learn English

C.it is very important for students to use good dictionaries properly

D.using dictionaries very often can’t help to improve writing小题2: According to the passage, which of the following is WRONG?

A.Dictionaries have little effect on learning to speak English.

B.Whatever new words you meet while reading, never use dictionaries.

C.Small two-language dictionaries have serious shortcomings.

D.Reading something for the first time, you’d better not use dictionaries.小题3: When in the reading does the writer advise students to use a good dictionary?

A.At the beginning of the reading

B.At the end of the reading

C.During the first reading

D.After the first reading小题4: This passage mainly tells us        .

A.that students shouldn’t use small two-language dictionaries

B.what were the shortcomings of small two-language dictionaries

C.why students should use large college edition dictionaries

D.what dictionary students should choose and how to use it

判断题