问题 问答题

用一根长20cm、劲度系数k=200N/m的弹簧水平拉着放在水平桌面上的质量为1kg的木块,弹簧的长度逐渐伸长到22.4cm时木块开始运动,当弹簧的长度为21.7cm时,木块在桌面上做匀速运动.求:(g=10N/kg)

(1)木块受到的最大静摩擦力多大?静摩擦力的变化范围怎样?

(2)木块与桌面间的动摩擦因数是多少?

答案

(1)物体所受的最大静摩擦力就等于物体刚开始滑动时的弹簧拉力大小,

由胡克定律可求得:F1=200×0.024N=4.8N,因此最大静摩擦力大小为4.8N,所以静摩擦力的变化范围为0-0.48N.

(2)当拉着物体匀速前进时,弹簧伸长1.7cm.则弹簧的拉力等于滑动摩擦力.

所以动摩擦因数为μ=

k△x
mg
=
200×0.017
10
=0.34

答:(1)木块受到的最大静摩擦力为0.48N,静摩擦力的变化范围为0-0.48N;

(2)木块与桌面间的动摩擦因数是0.34

单项选择题
阅读理解

The way people hold to the belief that a fun-filled,pain-free life equals happiness actually reduces their chances of ever attaining real happiness.If fun and pleasure are equal to happiness then pain must be equal to unhappiness.But in fact,the opposite is true:more often than not things that lead to happiness involve some pain.

As a result,many people avoid the very attempts that are the source of true happiness.They fear the pain inevitably brought by such things as marriage, raising children, professional achievement,religious commitment,self improvement.

Ask a bachelor (单身汉)why he resists marriage even though he finds dating to be less and less satisfying.If he is honest he will tell you that he is afraid of making a commitment.For commitment is in fact quite painful.The single life is filled with fun,adventure,excitement.Marriage has such moments,but they are not its most distinguishing features.

Couples with infant children are lucky to get a whole night’s sleep or a three-day vacation.I don’t know any parent who would choose the word fun to describe raising children.But couples who decide not to have children never know the joys of watching a child grow up or of playing with a grandchild.

Understanding and accepting that true happiness has nothing to do with fun is one of the most liberating realizations.It liberates time:now we can devote more hours to activities that can genuinely increase our happiness.It liberates money:buying that new car or those fancy clothes that will do nothing to increase our happiness now seems pointless.And it liberates us from envy:we now understand that all those who are always having so much fun actually may not be happy at all.

小题1:According to the author,a bachelor resists marriage chiefly because          

A.he finds more fun in dating than in marriage

B.he believes that life will be more cheerful if he remains single

C.he is reluctant to take on family responsibilities

D.he fears it will put an end to all his fun adventure and excitement小题2:Raising children,in the author’s opinion,is           

A.a moral duty

B.a thankless job

C.a source of inevitable pain

D.a rewarding task小题3:From the last paragraph,we learn that envy sometimes stems from       

A.hatred

B.misunderstanding

C.prejudice

D.ignorance小题4:To understand what true happiness is one must        

A.be able to distinguish happiness from fun

B.make every effort to liberate oneself from pain

C.put up with pain under all circumstances

D.have as much run as possible during one’s lifetime小题5:What is the author trying to tell us?

A.It is important to make commitments.

B.One must know how to attain happiness.

C.Happiness often goes hand in hand with pain

D.It is pain that leads to happiness.