问题 单项选择题

有一架不等臂天平,两个力臂分别为L1、L2,把物体放在左盘测得m1=90g,放在右盘测得m2=160g,则物体质量实际是()。

A.100g

B.120g

C.130g

D.70g

答案

参考答案:B

解析:

根据杠杆平衡原理,GL1=GL2,即mgL1=mgL2,通常情况下L1=L2,但此题所讲的天平是不等臂天平,所以两者并不相等。根据题意可知:物体放左盘时,mgL1=mgL2;物体放右盘时,mgL2=m'gL1,两式相乘并化简,得:m=,代入数值,得:m==120g,所以答案为B。

阅读理解

阅读理解。

     Today Newtown is a clean place, but many years ago there were millions of rats there. The rats even attacked (攻击) the cats and dogs. Sometimes many of them tried to bite men or women at night. The rats

were very large in size and they harmed (伤害) many people.The city office ordered every one to kill rats. However, most people were lazy, so they did not kill many rats.

     The city office could do nothing with the citizens and could do nothing with the rats, either. Some time

later, they had to pay some money for each dead rat. That made the people very happy. They at once

began to kill rats. They got as many dead rats as they could. And some of them even stopped their own

work to kill rats because they could get more pay. Every day a city official(官员) put all the dead rats

together. He was very busy doing that, because sometimes a man brought hundreds of them in one day.

Two weeks later, there were not so many rats in the city as before, but people still brought many dead rats to the city office. The city officials felt surprised at this but at last they found out people were stealing the

dead rats they had collected. So the city official had to order his men to dig a big hole and put the dead

rats in it. Soon there were no more rats and the city did not have to pay any more money for that.

1. The rats in Newtown were once ______

A. as big as cats

B. as dangerous as dogs

C. huge in size

D. run into cars there

2. When the city office first ordered the citizens to kill rats, most people ______.

A. had to pay for each dead rat

B. stole dead rats

C. were too lazy to kill rats

D. killed nearly all the rats very soon

3. A big hole was dug so that ______.

A. the rats could come out to attack people at night

B. people could take many rats

C. the people loved dead rats

D. people could not steal dead rats for money

多项选择题