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问题 选择题

神舟六号载人飞船2005年10月12日升空，在太空环绕地球飞行77圈后于10月17日顺利返回，这标志着我国航天事业又迈上了一个新台阶．假定正常运行的神舟六号飞船和通信卫星（同步卫星）做的都是匀速圆周运动．下列说法正确的是 （　　）

A．神舟六号飞船的线速度比通信卫星的线速度小

B．神舟六号飞船的角速度比通信卫星的角速度小

C．神舟六号飞船的运行周期比通信卫星的运行周期小

D．神舟六号飞船的向心加速度比通信卫星的向心加速度小

答案

根据万有引力提供向心力知G

mM
r2
=m
v2
r
=mrω2=mr
4π2
T2
=ma，抓住飞船在近地轨道上运行，轨道半径远小于同步卫星的轨道半径

A、v=

GM
r
，轨道半径大的线速度小，故A错误；

B、ω=

GM
r3
，轨道半径大的角速度小，故B错误；

C、T=

4π2r3
GM
，轨道半径大的周期大，故C正确；

D、a=

GM
r2
，轨道半径大的向心加速度小，故D错误．

故选C

单项选择题

The landscape of the Giant’s Causeway, lurking below the gaunt sea wall where the land ends, must have struck wonder into the hearts of the ancient Irish, who subsequently felt inspired and created legends about its builder: the giant Finn McCool. The Causeway Coast has an unparalleled display of geological formations representing volcanic activity during the Early Tertiary Period some 5060 million years ago. Its Tertiary lavas of the Antrim Plateau, covering some 3,800 sq km, represent the largest remaining lava plateau in Europe.
The Causeway is a mass of basalt columns packed tightly together. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Altogether there are 40, 000 of these stone columns, mostly hexagonal, but some are quadrangular, pentagonal, heptagonal and octagonal. The tallest ate about 40 feet high, and the solidified lava in the cliffs is 90 feet thick in places.
A fine circular walk will take you down to the Giant’s Causeway, past amphitheatres of stone columns and formations with fanciful names like the Honeycomb, the Wishing Well, and the Giant’s Granny, past a wooden staircase to Benbane Head, and back along the cliff-top. Further down the coast, the stunning Carrick-a-rede rope bridge spans a gaping chasm between the coast and a small island used by fishermen. The eighty-foot drop can be crossed via the swinging bridge—and is not for the faint-hearted!
The Giant’s Causeway and Causeway Coast site was inscribed as a World Heritage Site by UNESCO in 1986. The site is of outstanding universal value and meets the criteria set in the World Heritage Convention. Namely, it is an outstanding example representing major stages of the earth’s history including the record of life: significant on-going geological processes in landform developments, and significant geomorphic and physiographic features; moreover, it also contains superlative natural phenomena and areas of exceptional natural beauty and aesthetic importance.
Moyle District Council’s Causeway Visitor Centre is open daily all year round and located on the cliff top 1 km from the site. The Centre is the ideal starting point for walks along the coastal and cliff-top paths, providing all excellent range of visitor services. A 12-minute audio-visual presentation, with commentary available in 5 European languages, enables visitors to further explore the origins of the Giant s Causeway through local folklore and scientific theory, and highlights the many other attractions of the Causeway Coast and Glens of Antrim area.

The last sentence in Paragraph 3 implies that the trip on the swinging bridge is quite ______.

A．inviting

B．intimidating

C．eye-opening

D．heart-breaking
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问答题

设二维随机向量(X，Y)联合概率密度为

求：

Z=X+Y的概率密度。
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