题干

如图是吊车起吊货物的结构示意图,伸缩撑杆为圆弧状,工作时它对吊臂的支持力始终与吊臂垂直,使吊臂绕O点缓慢转动,从而将货物提起.下列说法正确的是(  )

 

A:吊臂是一省力杠杆,但要费距离

B:吊臂是一个费力杠杆,但可以省功

C:匀速顶起吊臂的过程中,伸缩撑杆支持力的力臂变小

D:匀速顶起吊臂的过程中,伸缩撑杆支持力渐渐变小

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答案(点此获取答案解析)

D

同类题2

阅读理解

    There are many differences between country life and city life in many ways. One person even does different things in the two places.

    Julia lived in the countryside, but one year she decided to visit the capital city to do some shopping and to see the sights. She stayed at a hotel near the central market. She had seldom been to the city before, and was very excited about what she would find.

    On the first morning of her visit, as she walked from the hotel to the market, she passed a beggar(乞丐). He was holding up a notice, which said, “Blind from birth. Please give generously(慷慨地).”

    Julia felt sorry for the blind beggar and she bent down and put a dollar coin into his bowl. “Thank you.” he said.

    The same thing happened again the following day.

    On the third day, however, Julia did not have a dollar coin. She had only fifty cents, so she dropped this into the beggar's bowl.

    “What have I done wrong?” the beggar said, “Why are you so stingy today? ”

    Julia was very surprised at what the beggar said. “How do you know I haven't given you a dollar?” she said, “If you're blind you can't know what coin I put into your bowl.”

    “Ah, ” explained the beggar, “The truth is that I'm not blind. I'm just looking after this place for the regular beggar while he's on holiday.”

    “On holiday?” Julia said, “And what exactly does your blind friend do on holiday?”

    “He goes into the countryside,” the man said, “and takes photographs. He's a very good photographer.”

同类题5

阅读下面的文言文,完成下列小题。

    崔衍,著,深州安平人。父伦,字叙。伦宝应二年以右庶子使吐蕃虏背约留二岁执伦至泾州逼为书约城中降伦不从更囚逻娑城阅六岁,终不屈,乃许还。

    衍,天宝末擢明经,调富平尉。继母李不慈,伦自吐蕃归,李敝衣以见,问故,曰:“衍不吾给。”伦怒,召衍,将袒而鞭之,衍涕泣无所陈。伦弟殷趋白:“衍所禀举送夫人所,尚何云!”伦悟,繇是谮无入。调清源令,劝民力田,怀附流亡,观察使马燧表其能,徙美原。父卒,事李益谨,岁为李子郃偿负不胜计,故官刺史,妻子仅免饥寒

    历苏、虢二州。虢居陕、华间,而赋数倍入,衍白太重。裴延龄领度支,方聚敛,私谓衍:“前刺史无发明,公当止。”衍不听,复奏:“州部多岩田,又邮传剧道,属岁无秋,民举流亡,不蠲减租额,人无生理。臣见长吏之患,在因循不以闻。不患陛下不忧恤也,患申请不实;不患朝廷不矜贷也。陛下拔臣大州,宁欲视民困而顾望不言哉?”德宗公其言,为诏度支减赋。迁宣歙池观察使,简静为百姓所怀。幕府奏聘皆有名士,后多显于时。卒,年六十九,赠工部尚书。

    衍俭约畏法,室无妾媵,禄稍周于亲族,葬埋嫁娶,倚以济者数十家。及卒,不能蕆丧,表诸朝,赐赙帛三百段,米粟称之。

    先是,天下以进奉结主恩,州藏耗竭,韦皋、刘赞、裴肃为之倡。赞死,衍代之。旧贡金锡凡十八品,皆倍直市于州,民匮,多逃去,衍至,蠲革之。居十年,啬用度,府库充衍。及穆赞代州,以钱四十万缗假民赋,故虽旱,人不流捐,由衍蓄积有素也。路应为观察使,以衍有惠在民,言状。元和元年,诏书褒美,曰懿。

(节选自《新唐书•列传第八十九》)