问题 选择题

如图所示,A为绕地球做椭圆轨道运动的卫星,B为地球同步卫星,P为A、B两轨道的交点。下列说法中正确的是(     )

A.卫星A所受万有引力完全提供向心力

B.卫星B相对地面静止,一定不会与A相撞

C.卫星B加速后其轨道可与卫星A轨道相同

D.卫星A在远地点加速后其轨道可以变成比B轨道半径更大的圆轨道

答案

答案:D

题目分析:如果卫星所受万有引力完全提供向心力,卫星将做匀速圆周运动,轨迹不可能为椭圆,所以A错;由于卫星为地球同步卫星,周期与地球自转周期相同,相对地面静止,但由于两颗卫星不再同一个轨道,有交叉点,所以两颗卫星有可能相撞,B错;卫星B加速后进入椭圆轨道,必须在A轨道的近地点加速,由此可知C错;同理卫星A在远地点加速后其轨道可以变成比B轨道半径更大的圆轨道,D对;

点评:本题难度较小,只有天体所受万有引力完全提供向心力时才能保证天体做匀速圆周运动

阅读理解

A recent research shows that physical activity(体育活动) may help students do better in their classes. The research comes because educators in some countries are reducing time for activities like P.E. class.

Researchers go over the results of fourteen studies. Twelve were from the United States, one from Canada and one from South Africa. The studies appeared between 2009 and 2013. They included more than 55,000 children, aged six to eighteen.

Thomas Singh, one of the researchers, says the studies showed a relationship between physical activity and scores on subjects such as math, English and reading. Ms. Singh offers some possible explanations (解释):

“ There are, first, physiological (生理的) explanations, like more blood flow, and so more oxygen (氧气) to the brain. Being physically active means there are more hormones( 荷尔蒙) produced like endorphins ( 脑内啡). And endorphins make your stress level lower and your mood improved, which means you will perform better in class. Also, students who take part in organized sports learn rules and how to follow them. This could improve their classroom behavior and help them know what they should do.

All in all, the finding tells us that physically active kids are more likely to do better in school. Ms. Singh says schools should consider the finding before they cut physical education programs. 

小题1:What led the researchers to do the studies?

A.Schools’cutting physical activity.

B.Their interest in physical activity.

C.The tests including about 55,000 children.

D.Kids’unsatisfying schooling performance.小题2:How many countries took part in the studies?

A.14.

B.12

C.2.

D.3.小题3:Which of the following is NOT one of the advantages of physical activity according to the passage?

A.Getting more blood flow.

B.Better classroom behavior.

C.Feeling relaxed and refreshed.

D.Greater ability to organize activities.

问答题 简答题