问题 问答题

一艘宇宙飞船绕着某行星作匀速圆周运动,已知运动的轨道半径为r,周期为T,引力常量为G,行星半径为R.求:

(1)行星的质量M;

(2)行星表面的重力加速度g;

(3)行星的第一宇宙速度v.

答案

(1)行星对飞船的万有引力提供飞船所需向心力

       G

Mm
r2
=m(
T
)2r

解得

      M=

4π2r3
GT2

故行星的质量为

4π2r3
GT2

(2)重力等于万有引力

      m′g=

GMm′
R2

解得

  g=

GM
R2
=
G
R2
?
4π2r3
GT2
=
4π2r3
R2T2

故行星表面的重力加速度为

4π2r3
R2T2

(3)卫星以第一宇宙速度绕行星附近作匀速圆周运动

    G

Mm
R2
=m
v2
R

解得

  v=

GM
R
=
G
R
?
4π2r3
GT2
=
4π2r3
RT2

故行星的第一宇宙速度为

4π2r3
RT2

阅读理解

I have nothing to say about whether or not "A Day Without Immigrants" had a positive or negative effect on the question of how the federal government should deal with the huge population of undocumented illegal immigrants in this country. I'll leave that question to others.

Regardless of how you feel about the event, one conclusion is obvious. Using the theme of" A Day Without" as a way to focus national attention on a particular subject is a great idea, and could serve as a method for increasing public awareness about other issues that affect daily life in America. Here are my top three examples:

"A Day Without Speeding." How hard could this be? If all drivers would strictly obey every posted speed limit on highways, downtown, on neighborhood streets, and in parking lots for one 24-hour cycle, the benefits would be huge.

"A Day Without Losing Your Temper." A higher degree of difficulty here, and I know that from personal experience. When something goes wrong at home or on the job, there may be brief satisfaction in yelling or kicking the furniture, but erupting with anger usually doesn't do anything to solve the problem, and in many cases it makes the situation more unpleasant.

"A Day Without Having All The Answers." I would like to get a break from hearing anyone speak the phrase, "Here's what you're doing wrong." Right now the US is jampacked with citizens who believe they can point out the cause of every single problem in existence and then solve it on the spot. I believe they need to be quiet sometimes and use their ears instead of their voices.

I wonder how would average Americans react to such a collective pause? Would it be a day of angry protests, or joyful celebrations?

小题1:The first paragraph suggests           .

A.the writer is against A Day Without Immigrants

B.immigrants are not equally treated by the US federal government

C.the writer cares little about the problems of the country.

D.some people move to America without the permission of the US government小题2:"The event" in the second paragraph is related to          .

A."A Day Without Immigrants"

B."A Day Without Having All The Answers"

C."A Day Without Speeding"

D."A Day Without Losing Your Temper"小题3:According to the writer, losing one's temper is           .

A.personal

B.satisfying

C.pointless

D.unpleasant小题4:The writer suggest a day without having all the answers because         .

A.he thinks the American are too proud

B.he doesn't think what he is doing is wrong

C.he wants others to keep quiet

D.the writer is so tired that he needs a break

单项选择题