问题 选择题

如图所示,传送带的水平部分长为L,传动速率为v,在其左端无初速度释放一小木块,若木块与传送带间的动摩擦因数为μ,则木块从左端运动到右端的时间可能是(    )

A.

B.

C.

D.

答案

ACD

题目分析:刚开始释放时,由于传送带的速度大于小物块的速度,因此,小物块在传送带上受重力mg、支持力N和滑动摩擦力f作用,显然,小物块要被加速,其加速度大小为μg,但小物块最终的速度不可能超过传送带的速度,因为当两者速度相等时,相对静止,摩擦力消失,小物块将匀速运动,因此若L=,则小物块一直匀加速至B端,且速度刚好等于v,其运动时间为,故选项D正确;若L<,则小物块一直匀加速至B端,速度小于v,其运动时间为,故选项C正确;若L>,则小物块先匀加速至速度为v,再以速度v匀速运动至B端,其运动时间为,故选项A正确;小物块不可能从A匀速运动至B端,即时间不可能为,故选项B错误。

问答题 简答题
阅读理解

Roughly speaking, teachers in traditional education may feel that students are too young to know much about the world, and have to be told what to do most of the time. By contrast, open education offers teachers quite different feelings. Students are expected to be responsible for their own education—to discover subjects by themselves instead of being made to study them. This way of teaching allows the students to grow independently and to develop their own interests in many subjects. Some students who do badly in a traditional classroom will be happier in an open classroom and enjoy learning. They will not have to worry about grades or rules at all.

However, many students won’t do well in an open classroom. For these students, they will do little in school if there are too few rules. They will not make good use of open education, which is so different from traditional class, because they may have a problem getting used to making so many choices on their own. For them it is important to have some rules to direct them. They may worry about the rules even when there are no rules. In addition to that, some traditional teachers do not believe in open education and do not like it either.

Since both of some good points and bad points of the open education have been explained clearly, you may have your own opinion on the issue. Personally, I think that the concept of open education is good only in theory. In actual fact, it may not work very well in a real class or school. I believe, most students, but of course not all students, need some structure in their classes. They may want and need to have rules.

In some cases, they must be made to study some subjects. Many students are pleased to find subjects they have to study interesting. They would not study those subjects if they did not have to.

小题1:Open education allows the students to_________________.

A.grow as the educated

B.be responsible for their life

C.discover subjects outside class

D.develop their own interests小题2:Open education may be a good idea for the students who __________.

A.worry about grades and rules

B.aren’t used to making choices

C.do well in a traditional classroom

D.enjoy having some rules in class小题3:Some students will do little in an open classroom because___________.

A.there are too many rules

B.they don’t like activities

C.there are too few rules

D.they worry about the rules小题4:Which of the following best summarizes the passage?

A.Open education is a really complex idea.

B.Open education is better than traditional education.

C.Traditional teachers don’t believe in open education.

D.The writer thinks open education is practical.