问题 选择题

在一种叫“蹦极跳“的运动中,质量为m的游戏者,身系一根长为L,弹性优良的轻质柔软橡皮绳,从高处由静止开始下落1.5L时到达最低点,若在下落过程中不计空气阻力,则下列说法正确的是(  )

A.速度先增大后减小

B.加速度先增大后减小

C.动能增加了mgL

D.重力势能减小了mgL

答案

AB、人开始时只受重力,则在绳张紧之前人做自由落体运动,速度增加,动能增加;

绳张紧后,绳的弹力开始增大,但开始时仍小于重力,故人继续加速,直到弹力等于人的重力;

此后,人受到的弹力大于重力,故人开始减速,速度减小;绳子继续伸长,弹力越来越大,合力向上越来越大,故加速度也越来越大;

由以上分析可知,人的速度是先增大后减小;加速度先保持不变,然后减小,后又增大,故A正确,B错误;

C、在人下落的最后,人的速度为零,故动能的变化为零,故C错误;

D、人下落的高度大于L,故重力做功大于mgL,故重力势能减小量大于mgL,故D错误;

故选A.

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 (预担的义务), 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 is reluctant to take on family responsibilities

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

C.he finds more fun in dating than in marriage

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 rewarding task

D.a source of inevitable pain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.have as much fun as possible during one’s lifetime

B.make every effort to liberate oneself from pain

C.put up with pain under all circumstances

D.be able to distinguish happiness from fun小题5:What is the author trying to tell us?

A.Happiness often goes hand in hand with pain.

B.One must know how to attain happiness.

C.It is important to make commitments.

D.It is pain that leads to happiness.

单项选择题