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如图所示，在建筑工地上有一台塔式起重机，其部分技术参数如下表所示．

吊臂远端起重质量（㎏）700
最大起重质量（㎏）3000
最大提升速度（m/s）0.6
独立使用高度（m）30.5
平衡块质量（㎏）6000
（1）该起重机一次最多能吊起的建筑材料重力为多少N？吊起的建筑材料达到最大起重量时，建筑材料到竖直塔的距离是近端还是远端？

（2）该起重机将重为2×104N的建筑材料以最大提升速度吊起6m，起重机对建筑材料做了多少有用功？如果起重机的实际总功率为25kW，起重机的机械效率是多大？

答案

（1）起重机一次吊起的最大起重建筑材料的重力G最大=m最大g=3000kg×10N/kg=3×104N；

根据杠杆的平衡条件F1L1=F2L2可知，平衡块质量和平衡块到塔的距离一定时，吊起的建筑材料质量为最大时，建筑材料到塔的距离最近；

（2）①起重机吊起重为2×104N的建筑材料时，

做的有用功W有用=Gh=2×104N×6m=1.2×105J；

②起重机的工作时间：t=

s
v
=
6m
0.6m/s
=10s；

起重机做的总功W总=Pt=25×103W×10s=2.5×105J；

起重机的机械效率η=

W有用
W总
×100%=
1.2×105J
2.5×105J
×100%=48%．

答：（1）起重机吊起的建筑材料的重力为3×104N；吊起的建筑材料达到最大起重量时，建筑材料到塔的距离是最近的；

（2）起重机做的有用功为1.2×105J；起重机的机械效率为48%．

单项选择题

With the spread of inter-active electronic media a man alone in his own home will never have been so well placed to fill the inexplicable mental space between cradle and crematorium. So I suspect that books will be pushed more and more into those moments of travel or difficult defecation (1) people still don’t quite know what to do with.

When people do read, I think they’ll want to feel they are reading literature, or (2) something serious. (3) you’re going to find fewer books presenting themselves as no- nonsense and (4) assuming literary pretensions and being packaged as works of art. We can expect an extraordinary variety of genre, but with an underlying (5) of sentiment and vision.

Translators can only (6) from this desire for the presumably sophisticated. We can look forward to lots of difficult names and fantastic stories of foreign parts enthusiastically (7) by the overall worship of the "global village" Much of this will be awful and some wonderful, (8) don’t expect the press or the organizers of prizes to offer you much help in making the appropriate distinctions. They will be chiefly (9) in creating celebrity, the greatest enemy of discrimination, but a good prop for the (10) consumer.

Every ethnic grouping over the world will have to be seen to have a great writer--a phenomenon that will (11) a new kind of provincialism, more chronological than geographic, (12) only the strictly contemporary is talked about and (13) . Universities, including Cambridge, will include (14) their literature syllabus novels written only last year. (15) occasional exhumation for the Nobel, the achievements of ten or only five years ago will be largely forgotten.

In short, you can’t go too far wrong when predicting more of the same. But there is a (16) side to this--the inevitable reaction against it. The practical things I would like to see happen--publishers seeking less to (17) celebrity through extravagant advertising, (18) and magazines (19) space to reflective pieces--are rather more improbable than the Second Coming (耶稣复临). But dullness never quite darkens the whole planet. In their own idiosyncratic fashion a few writers will (20) be looking for new departures.

17（）

A.prevent

B.explore

C.prohibit

D.generate
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填空题

车辆路试一般是在（）上装载75％的规定载荷、时速（）30km/h的情况下进行约15km的行车试验。
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