问题 单项选择题

在位移法典型方程的系数和自由项中,数值范围可为正、负实数的有()。

A.主系数

B.主系数和副系数

C.主系数和自由项

D.副系数和自由项

答案

参考答案:D

解析:

在位移法典型方程中,主系数rii代表由单位位移Zi=1的作用所引起的在Zi自身方向上的约束反力,它必定与该单位位移的方向一致,故是恒正的;而副系数rii(i≠j)代表由单位位移Zj=1的作用所引起的在Zi方向上的约束反力,它可以与所设定的 Zi同向、反向,或者是无该项约束反力发生,所以它可能为正、为负或者为零。同理,自由项也可能为正、为负或者为零。因此,选项(D) 正确。

[点评] 本题旨在考察位移法典型方程中各项系数与自由项的物理意义,只有真正理解其意义,才能体会位移法典型方程的实质。

阅读理解

阅读理解。

     What do people do with their old, out-of-date but still useful computers? Most people don't know what to

do with them. Many old computers are put away in homes. Many more are simply thrown out with the rubbish.   Some companies are thinking of ways to bring down the number of old computers. Sony has agreed to help

recycle old Sony products. Dell, Hewlett-Packard and other companies now also take back some old computers.        In some countries, laws are being passed, too. Computer companies will have to collect and recycle their

used products. And 70% of computer waste must be recycled. The idea behind the laws is that computer

companies themselves should pay the cost. That will encourage them to make computers that are easier and

cheaper to repair and upgrade (升级).  

     Yet while many people are throwing away good computers, others cannot afford them at all. Hundreds of

organizations are working to solve this problem. They collect and repair old computers. Some also teach others

how to repair computers.  

     The computers then go to schools, charities and people who need them. Giving a used computer to one

of these organizations can turn one person's rubbish into someone else's useful things-and cut down on waste,

too.  

1. How do people usually deal with their old computers?  

A. They stop using them.

B. They give them to others.

C. They fix them up.  

2. What do some computer companies do to bring down the number of old computers?   

A. They don't do anything about them.  

B. They take back some old ones.   

C. They donate them to some charities

3. According to some countries' laws, what must computer companies do?    

A. Collect and recycle most of computer waste.   

B. Repair and upgrade old computers.   

C. Make much cheaper computers.

4. What may people who can't afford computers think of the used computers?  

A. Expensive.

B. Useful.

C. Out-of-date. 

5. What is the passage about?  

A. Repairing old computers.  

B. Selling old computers.   

C. Recycling old computers.

名词解释