问题 选择题

2010年3月5日12时55分,我国在酒泉卫星发射中心用“长征四号丙”运载火箭成功地将“遥感卫星九号”送入太空预定轨道.卫星的运动轨道为一椭圆,如图所示,地球的球心位于该椭圆的一个焦点上,A、B两点分别是卫星运行轨道上的近地点和远地点.若A点在地面附近,且卫星所受阻力可以忽略不计,则下列说法中正确的组合为(  )

①运动到B点时其速率可能等于7.9km/s

②运动到A点时其速率一定大于7.9km/s

③若要卫星在B点所在的高度做匀速圆周运动,需在B点加速 ④.若要卫星在B点所在的高度做匀速圆周运动,需在A点加速.

A.①③

B.①④

C.②③

D.②④

答案

①②在A点若做圆周运动,则速度等于第一宇宙速度,为7.9km/s,因为在A点做离心运动,知A点的速度大于7.9km/s.根据G

Mm
r2
=m
v2
r
知,v=
GM
r
,则轨道半径越大,做圆周运动的线速度越小,在B点若做圆周运动,知线速度一定小于7.9km/s,现在在B点做近心运动,速度一定小于7.9km/s.故①错误,②正确.

③④、在B点由于速度较小,万有引力大于所需的向心力,做近心运动,知欲做圆周运动,必须加速,使得万有引力提供向心力.故③④错误.

故选C.

阅读理解

I won’t forget her advice

I had a very special teacher in high school many years ago. Her husband died of a heart attack. About a week after his death, she shared some of her insights(见解)with us.

The class was nearly over. She said, “Before class is over, I would like to share with all of you an important thought.”

“Each of us is put here on Earth to learn, share, love, enjoy and give all by ourselves. We must make the best of every single day.”

Her eyes were beginning to water and she went on, “So I would like you all to make me a promise. From now on, on your way to school, or on your way home, find something beautiful to notice. It doesn’t have to be something you see — it could be a nice smell — perhaps of freshly baked bread, or it could be the sound of the breeze slightly blowing the leaves in the trees.”

“Please look for these things, and love them. For, although it may seem common, these things are the stuff(素材)of life. They are the little things we are put here on Earth to enjoy. They are the things we often take for granted. We must make it a point to notice them, for at any time it can all be taken away.”

The class was completely quiet. We picked up our books and went out of the room silently. That afternoon, I noticed more things on my way home from school than I had that whole term.

I usually think of that teacher and try to enjoy all of those things that we sometimes overlook (忽略).

Take notice of something special you see during your lunch hour today. Walk barefoot along the beach at sunset. For as we get older, it is not the things we did that we regret(后悔), but the things we didn’t do.

小题1:What happened to the teacher’s husband?

A.He was ill.

B.He died of a heart attack.

C.He made a promise.

D.He became a teacher, too.小题2:The high school teacher shared her insights with her students ____________.

A.on her way to school

B.on her way home

C.at the beginning of class

D.at the end of class小题3:The underlined word “breeze” (in Paragraph 4) means ____________?

A.colourful cloud

B.white snow

C.light wind

D.heavy rain小题4:What is the teacher’s advice?

A.We should love life and enjoy things around us.

B.We should share our important thoughts with others.

C.We should never feel sorry for what we have done.

D.We should do importand things and overlook little ones.

多项选择题 X型题