问题 选择题
电气化铁路的输电线常用图所示的方式悬挂在钢缆上.钢缆的A端固定在电杆上,B端连接在滑轮组上.滑轮组由a、b、c三个滑轮构成,配重P由多个混凝土圆盘悬挂在一起组成,配重的总重为G,若不计摩擦和滑轮的重量,则以下说法中正确的是(  )

A.a为动滑轮,B端钢缆受到滑轮组的拉力大小约为3G

B.a为动滑轮,B端钢缆受到滑轮组的拉力大小约为G

C.a、c为动滑轮,B端钢缆受到滑轮组的拉力大小约为3G

D.a、c为动滑轮,B端钢缆受到滑轮组的拉力大小约为G

答案

答案:A

单项选择题 A1/A2型题
问答题

In the past year, a lot has changed in the field of human spaceflight. (46)In January, President George Bush brushed aside the fact that America’s entire space-shuttle fleet was grounded when he announced grandiose plans to put people back on the moon, and then to launch a manned mission to Mars. (47) In June, Burt Rutan, an American aeronautical engineer, showed that human spaceflight was no longer the preserve of governments by sending a man to the edge of space in Space Ship One, a privately financed vehicle that cost about the same to build as a luxury yacht. That was followed in September by Sir Richard Branson, the British entrepreneur behind the Virgin brand, announcing that he had signed a deal with Mr. Rutan to work on plans for a fleet of five suborbital vehicles developed from Space Ship One.

(48) Now, in the dying days of the year, America’s Congress has passed a bill that unravels a tangle about who would be responsible for regulating the fledgling industry, and under what terms. (49) The bill also allows passengers to fly, on the understanding that this new generation of vehicles may not be as safe as taking a commercial flight between, say, New York and London.

The official line from Virgin Galactica, as Sir Richard’s latest venture is modestly named, is that this coming change in the law makes no practical difference to the firm’s plans, since they do not intend to fly unless they can make their spacecraft as safe as a private jet. But it must surely come as some sort of relief. In any case, Will Whitehorn, director of corporate affairs at Virgin’s headquarters in London, and soon to become the president of Virgin Galactica, says that work is under way on a mock-up of the interior of a new spacecraft that will hold five passengers. (50) Virgin has already committed $20m towards licensing the SpaceShipOne technology from Mr. Rutan and his financial backer Paul Allen, a software billionaire.

(47) In June, Burt Rutan, an American aeronautical engineer, showed that human spaceflight was no longer the preserve of governments by sending a man to the edge of space in Space Ship One, a privately financed vehicle that cost about the same to build as a luxury yacht.