问题 选择题

极地卫星(轨道平面经过地球的南北两极)圆轨道的半径为r,周期为2h.赤道卫星9轨道平面为赤道平面)圆轨道半径为4r.则两卫星从距离最近到下一次最近的时间为(  )

A.

16
7
h

B.14h

C.16h

D.30h

答案

卫星绕地球做圆周运动由万有引力提供向心力,据此有:

G

mM
R2
=mR
4π2
T2
可得:由此可知,赤道卫星轨道半径是极地卫星轨道半径的4倍,其运行周期是极地轨道卫星周期的8倍,即

赤道卫星的周期T2=8T1=16h

因为卫星做圆周运动,相距最近位置根据圆周的对称性可知,有两个位置,其两位置间的时间差为每个卫星周期的

T
2
,因为极地卫星周期为2h,其半周期为1h,赤道卫星周期为16h,其半周期为8h,所以在赤道卫星运转
T
2
=8h的过程中,极地卫星运转刚好是4个周期,故由第一次相距最近到第二次相距最近的时间为赤道卫星运转1个周期的时间即t=16h.

故选C.

阅读理解

In an ideal world, people would not test medicines on animals. Such experiments are stressful and sometimes painful for animals, and expensive and time–consuming for people. Yet animal experimentation is still needed to help bridge vast gaps in medical knowledge. That is why there are some 50 to 100 million animals used in research around the world each year.

Europe, on the whole, has the world’s most restrictive laws on animal experiments. Even so, its scientists use some 12 million animals a year, most of them mice and rats, for medical research. Official statistics show that just 1.1 million animals are used in research in America each year. But that is misleading. The American authorities do not think mice and rats are worth counting and, as these are the most common laboratory animals, the true figure is much higher. Japan and China have even less comprehensive data than America.

Now Europe is reforming the rules governing animal experiments by restricting the number of animals used in labs. Alternatives to animal testing, such as using human tissue or computer models, are now strongly recommended. In addition, sharing all research results freely should help to reduce the number of animals for scientific use. At present, scientists often share only the results of successful experiments. If their findings do not fit the hypothesis being tested, the work never sees the light of day. This practice means wasting time, money, and animals’ lives in endlessly repeating the failed experiments.

Animal experimentation has taught humanity a great deal and saved countless lives. It needs to continue, even if that means animals sometimes suffer. Europe’s new measures should eventually both reduce the number of animals used in experiments and improve the way in which scientific research is conducted.

小题1:What is the main idea of this passage?

A.The success of animal experiments should be ensured.

B.A ban on the use of animals in the lab should be enforced.

C.Greater efforts need to be taken to reduce the number of lab animals.

D.Scientists should be required to share their research results with each other.小题2:Which of the following statements is true about animals used in the lab?

A.America uses only about 1.1 million lab animals per year.

B.Europe does not use mice and rats as lab animals at all.

C.Britain does not use as many lab animals as China does.

D.Japan has less comprehensive data on the number of lab animals used each year.小题3:Which of the following is mentioned as an alternative to replace animal experiments?

A.Statistical studies.

B.Computer models.

C.DNA planted in animals.

D.Tissue from dead animals.小题4:What usually happens to unsuccessful animal experiments?

A.They are not revealed to the public.

B.They are made into teaching materials.

C.They are collected for future publication.

D.They are not removed from the research topic list.

单项选择题