问题 选择题

如图所示,一颗行星和一颗彗星绕同一恒星的运行轨道分别为A和B,A是半径为r的圆轨道,B为椭圆轨道,椭圆长轴QQ′为2r。P点为两轨道的交点,以下说法正确的是

A.彗星和行星经过P点时受到的万有引力相等

B.彗星和行星绕恒星运动的周期相同

C.彗星和行星经过P点时的速度相同

D.彗星在Q′处加速度为行星加速度的1/4

答案

答案:B

题目分析:据题意,行星和彗星质量未知,不能比较在P点处所受万有引力,A选项错误,根据开普勒第三定律,行星和彗星围绕同一中心天体,且半长轴相同,故周期相同,B选项正确;彗星和行星经过P点时的速度大小和方向均不同,C选项错误;彗星在Q点处与恒星球心的距离小于2r,加速度大于行星加速度的1/4,故D选项错误。

单项选择题

The road to controlling population growth in the 20th century was paved with good intentions and unpleasant policies that did not work, a new book argues an historian who grew up as the youngest of eight children might well be expected to approach the question of whether the world is overpopulated from an unusual angle. Matthew Connelly, a professor at Columbia University, dedicates his study of those who thought the planet had too many people and tried to do something about it to his parents, "for having so many children".
Yet, he assures the reader, it Was not his personal experience of large families that drew him to the subject. Mr. Connelly’s mentor, Paul Kennedy of Yale University, believed it was necessary to look beyond great-power rivalries to understand the post-cold-war era. In 1994 the pair wrote an article for Atlantic Mouthly arguing that population growth in poor countries, increasing awareness of global economic inequality and the prospect of mass migration could lead to clashes between the West and "the rest".
When, years later, Mr. Connelly began his own book on population growth, he still thought of the topic as a way to offer a broader understanding of world security. He ended up writing a very different-and angry-book, one about people who looked at the human race reproducing itself and saw what a gardener sees when looking at a prize plant: something to be encouraged to bloom in some places and pruned in others.
As the world population soared, the population controllers came to believe they were fighting a war, and there would be collateral damage. Millions of devices were exported to poor countries although they were known to cause infections and sterility. "Perhaps the individual patient is expendable in the general scheme of things," said a participant at a conference on the devices organized in 1962 by the Population Council, a research institute founded by John [D] Rockefeller, "particularly if the infection she acquires is sterilizing but not lethal. "
Furthermore, statistical estimates suggest that as much as 90% of the reason that women have families of a particular size is simply because that is the number of children they want. Where women gained education and rights, birth rates fell. As with reproduction itself, for people to become less fruitful, desire must precede performance.

The word "pruned" ( Line 5, Para. 3) most probably means______.

A.to foster the growth of

B.to remove from, esp. by cutting off

C.to inspire with the courage or confidence

D.to express p disapproval of

单项选择题