问题 计算题

(12分)如图所示,A、B叠放在光滑水平地面上,B与自由长度为L0的轻弹簧相连.当系统振动时,A、B始终无相对滑动,已知mA=3m,mB=m,当振子距平衡位置的位移x=时,系统的加速度为a,求A、B间摩擦力Ff与位移x的函数关系.

答案

Ff=-x

设弹簧的劲度系数为k,以A、B整体为研究对象,系统在水平方向上做简谐运动,其中弹簧的弹力为系统的回复力,所以对系统运动到距平衡位置为时,有k=(mA+mB)a,由此可得k=

当系统的位移为x时,A、B间的静摩擦力为Ff,此时A、B具有共同加速度a′,对系统有-kx=(mA+mB)a′②

对A有Ff=mAa′③

由①②③得Ff=-x.

阅读理解

When asked to point out one or two things that are most important to themselves, many put friends ahead of homes, jobs, clothes and cars.

A true friendship carries a long history of experience that determines who we are and keeps us connected. It is a treasure we should protect. Unfortunately, the better friends you are, the more probably you’ll have disagreements. And the result can be what you don’t want ---an end to the relationship.

The good news is that most troubled friendships can be mended. First, don’t let your pride get in your way. Most of us can forgive each other when differences are brought out in the open. Second, apologize when your’re wrong---even if you’ve been wronged. Over the course of a friendship, even the best people make mistakes. Sometimes, it may be best if the wronged person takes the lead and apologizes. When you apologize, give your friend a chance to admit that he has been wrong also. Third, see things from your friend’s point of view. And finally, accept that friendships change as our needs and lifestyle change. Making friends can sometimes seem easy. The hard part is keeping the connections strong during the natural ups and downs that have an effect on all relationships. My suggestion: consider friendship an honour and a gift, and worth the effort to treasure and nurture.

小题1:What is likely to happen if you have disagreements with your friends?

A.An end to the friendships.

B.Getting experience for the later life.

C.Quarrelling with each other.

D.Sitting down and have a talk.小题2:The author wrote the article mainly to tell us ________.

A.how to make friends

B.the benefits of making friends

C.what is a true friendship

D.how to mend a troubled friendship小题3:The underlined word “nurture” probably means ________.

A.mend

B.develop

C.keep

D.accept小题4:What causes the change of friendships?

A.Our characters.

B.Our hobbies.

C.Our living habits.

D.Our needs and lifestyle.小题5:Which of the following is True according to the passage?

A.A true friend can never have a disagreement with each other.

B.Most troubled friendships can be mended.

C.You don’t need to apologize when you know you are wronged.

D.Friends are more important than jobs, homes and cars.

单项选择题