问题 选择题

如图所示,表面光滑为R的半球固定在水平地面上,球心O的正上方Oˊ处有一个无摩擦定滑轮,轻质细绳两端各系一个小球挂在定滑轮上,两小球平衡时,若滑轮两侧细绳的长度分别为l1=2.4R,l2=2.5R.则这两个小球的质量之比m1:m2为(不计小球大小)(  )

A.24:1

B.25:1

C.24:25

D.25:24

答案

先以左侧小球为研究对象,分析受力情况:重力m1g、绳子的拉力T和半球的支持力N,作出力图.由平衡条件得知,拉力T和支持力N的合力与重力mg大小相等、方向相反.设OO′=h,根据三角形相似得:

     

T
l1
=
m1g
h
,得m1g=
Th
l1
…①

同理,以右侧小球为研究对象,得m2g=

Th
l2
…②

由①:②得

    m1:m2=l2:l1=25:24

故选D

阅读理解

We are all busy talking about and using the Internet, but how many of us know about the history of the Internet?

  Many people are surprised when they find that the Internet was set up(建立) in the 1960s. At that time, computers were large and expensive. Computer networks(网络) didn’t work well. If one computer in the network broke down, then the whole network stopped. So a new network system(系统) had to be set up. It should be good enough to be used by many different computers. If part of the network was not working, information could be sent through another part. In this way the computer network system would keep on working all the time.

  At first, the Internet was only used by the government, but, in the early 1970s, universities, hospitals and banks were allowed to use it too. However, computers were still expensive and the Internet was difficult to use. By the start of the 1990s, computers become cheaper and easier to use. Scientists had also developed software(软件) that made “surfing(浏览)” the Internet more convenient (方便).

  Today it is easy to get on-line and it is said that millions of people use the Internet every day. Sending e-mail is more and more popular among students.

The Internet has now become one of the most important parts of people’s life.

根据短文内容,选择正确答案(10分)

小题1:The Internet has a history of ______ years.

A、about 40             B、less than 30         C、more than 45

小题2:Scientists set up a new network system to ______.

A、make the computer cheaper

B、make the system work well

C、make the computer go well

小题3:The Internet was widely used in the ______.

A、1960s                B、1970s                C、1990s

小题4:The underlined words “get on-line” in Chinese mean ______.

A、上机               B、上网                 C、接线

小题5:Which of the following is NOT true?

A、People had enough softwares to get on-line fifteen years ago

B、Computers are much cheaper than before.

C、Today the Internet is used everywhere.

选择题