问题 计算题

(10分)如图所示,两个质量均为m的小环套在一水平放置的粗糙长杆上,两根长度均为l的轻绳一端系在小环上,另一端系在质量为M的木块上,两个小环之间的距离也为l,小环保持静止。试求:

  

(1)小环对杆的压力;

(2)小环与杆之间的动摩擦因数μ至少为多大?

答案

(1)FN=M g+mg (2)μ=

(1)整体法分析有:2FN=(M+2m)g        (2分)

即FN=M g+mg   由牛顿第三定律得:小环对杆的压力FN=M g+mg (2分)

(2)研究M得: 2FTcos300=Mg                 (2分)              

临界状态,此时小环受到的静摩擦力达到最大值,则有:

2FTsin300=μFN                                (2分)

解得:动摩擦因数μ至少为  μ=     (2分)

本题考查受力平衡问题,先以整体为研究对象,由受力分析可知两个小环受到的支持力等于整体受到的支持力,从而求得一个环所受支持力大小,以M为研究对象受力分析后求出绳子拉力大小,再以m为研究对象分析受力情况,可知绳子向左的分力等于最大静摩擦力大小,从而求得动摩擦因数

单项选择题
阅读理解

Now those same people perhaps come to realize that their kids are born with their own sets of DNA and personality traits(特点),and all you can do is loving and accepting them.As parents,throughout their growing years and beyond that, we need to be our kids' best cheerleaders,guiding them and helping them find their way.

I have stopped asking my son to sit down and concentrate.Obviously, he is concentrating just in his own way and not mine.We need to learn to accept our kids' ways of doing things.Some way may have worked for me but doesn't mean we need to carry it through generations.There is nothing sweeter than being individual(个体的) and unique(独特的).It makes us free and happy and that's just the way I want my kids to live their own life.

小题1:At the beginning, the author tried to keep his son seated in order to make him________.

A.pay more attention to his study.

B.keep silent in the room.

C.finish his homework on time.

D.get right answers.小题2:Inspired by the case of his son,the author began to doubt________.

A.the importance of parents.

B.the old form of education.

C.the relationship between kids and their parents.

D.the good grades of some kids.小题3:According to the passage, which of the following statements is right?

A. We should help kids correct their wrong ways.

B. Parents should study their kids’ DNA.    

C. Kids should be taught to behave themselves.

D. Parents should love and accept their kids.

小题4:The author has stopped asking his son to sit down because __________.

A.he has changed his traditional ideas.

B.his son wants to be unique.

C.his son doesn’t follow his advice.

D.his son’s DNA is different.小题5:Which of the following would be the best title of the passage?

A.Study hard and you'll be successful.

B.Be friendly to your children.

C.Children's success in their own style.

D.Parents' help with their children's study.