问题 填空题

如图所示,三个半径均为r、质量均为m的小球紧靠着由静止起沿倾角为α的光滑斜面下滑,斜面与光滑水平面间有光滑小圆弧连接,开始时第一个小球的最低点离水平面的高度为h.则第三个小球滑到水平面时的速度大小v3=______,在此过程中第二个小球对第一个小球所做的功W=______.

答案

把三个小球看成一个整体,整体滑到水平面时,高度下降了H=h+2rsinα,

根据动能定理得:

1
2
?3m?v2=3mgH

解得:v=

2gh+4grsinα

隔离第一个小球,由动能定理得:

W+mgh=

1
2
mv2

解得:W=2mgrsinα,

故答案为:

2gh+4grsinα
,2mgrsinα

阅读理解

阅读理解

Today almost everyone knows computers and the Internet. If I ask you “What is the most important in your life?” maybe you will say “Computers and the Internet.”

The first computer was made in 1946. it was very big but it worked slowly. Today computers are getting smaller and smaller. But they work faster and faster. What can computers do? A writer has said, “People can’t live without computers today.”

The Internet came a little later than computers. It is about twenty-five years later than computers. But now it can be found almost everywhere. We can use it to read books, writer letters, do shopping, play computer games or make friends.

Many students like the Internet very much. They often go into the Internet as soon as they are free. They make friends on the Internet and maybe they have never seen these friends. They don’t know their real names, ages, and even sex. They are so interested in making the “unreal friends” that they can’t put their hearts into study. Many of them can’t catch up with others on many subjects because of that.

We can use computers and the Internet to learn more about the world. But at the same time, we should remember that not all the things can be done by computers and the Internet.

小题1: The Internet can not be used for______.

A.studying

B.shopping

C.thinking

D.playing小题2: When the computer was invented, it was______.

A.large and worked quickly

B.small and worked slowly

C.large but worked slowly

D.small but worked quickly小题3: The Internet was born in about________.

A.1960

B.1970

C.1980

D.1985小题4:Which of the following is TRUE?

A.Few students like going into the Internet.

B.Students use the Internet to make “unreal friends.”

C.These “Unreal friends” often meet each other

D.Students know the friends on the Internet very well小题5:What does the writer think of the Internet?

A.It is wonderful

B.It can make students study harder

C.It is not good for students

D.It is helpful, but we can’t do everything on it.

单项选择题