问题 计算题

如图所示,两根平行金属导轨与水平面间的夹角为θ=37°,导轨间距为l=0.50 m,金属杆ab、cd的质量均为m=1 kg,电阻均为r=0.10 Ω,垂直于导轨水平放置.整个装置处于匀强磁场中,磁场方向垂直于导轨平面向上,磁感应强度B=2.0 T.用平行于导轨方向的拉力拉着ab杆匀速向上运动,两杆与斜面间的动摩擦因数μ=0.3,不计导轨电阻,重力加速度g=10 m/s2,sin 37°=0.6,cos 37°=0.8.则:要使cd杆静止在斜面上,拉力应为多大?

答案

12 N≤F≤16.8 N

将两杆看作整体,受力分析如图所示,

其中Ff为ab杆和cd杆受到的摩擦力.

当cd杆有向上的运动趋势时,Ffcd方向沿斜面向下,F1=2mgsin θ+μmgcos θ+FfcdFfcd有最大值μmgcos θ时,F1也有最大值Fmax=2mg(sin θ+μcos θ)=16.8 N

当cd杆有向下的运动趋势时,Ffcd方向沿斜面向上,

F2=2mgsin θ+μmgcos θ-Ffcd

Ffcd有最大值μmgcos θ时,F2有最小值

Fmin=2mgsin θ=12 N.

阅读理解与欣赏

阅读下面的作品。

在风中长大                朱以撒

①说风,可以从我小时候居住的环境追溯过来。这个滨海小城,走几步就可以看到逐排推动的雪浪花;而城市的另一面,则是终年绿意披拂的高山。这个小城的古典气味,就在海风和山风的冲兑下回旋,漾来漾去。

②小城人家安然地度着夏日,每人一把蒲扇,指掌轻轻收住扇把,左右摇动。一个没有任何降温设备的居家生活,从夏日里探到了它的朴素和简单,同时充满了对于气候轮转的乐于接受,还有婉约的调整,调整到稍稍适应即可,用一把充满草香的蒲扇。这与如今终日在写字楼内,空调的制动使整座大楼冷飕飕不同,白领可以在夏日穿着笔挺的西服,却不知,一个人不感受夏日之炎热,是辜负了这个时节固有的赏赐。

③总是在晚饭的时候,借助夕阳的余晖品尝,每一口饭和菜,都充满芳香。一盏忽忽悠悠的煤油灯摆上了桌,火舌温柔、委婉,昏黄暗淡,却可以照见一家老小。在摇曳的火舌下,厨房里是母亲熟练运动着的双手,碗碟正在被涮洗,暗中反射着寒光。没有电灯通明的老宅,简陋中透着温馨,是一种干稻草堆那般的温暖。

④有一些人注定要离乡背井,到更广阔的空间接受风雨的扑打。我从风中的山野来到这个省会城市,已经二十六岁,又高又瘦如风中之丝。城市是乡村的怪胎,许多的空旷越来越紧密地被高层建筑挤占,似乎不占尽空间就不足以称城市。一座高大的建筑就是一面挡风的盾牌,盾牌多了,激荡的风难以穿越。城市闷热起来,蒲扇已经消失,电器降温设备在炎炎夏日的居室里制造着非自然的情调。有时也能巧遇故旧,当我放弃大路钻入小巷,风突然旋转而至,皮肤准确地判断它来自高空或者原野。即使城市生活久了,皮肤的感觉依旧,像一小节细微的芒刺拂动,有一点幸福的异样。

⑤台风是这个城市最大的威胁。当我们看到台风时,已经是这股力量穿行过后留下的破坏痕迹了——楼房倒塌,桥梁倾圮,林木拔起,船只颠覆。装点城市的无数大型广告,妖冶的美女花容失色,断成几节,只余一只煽情的眼。每一次台风过去,城市惊魂未定。对于大自然而言,只不过是它一次急促的呼吸罢了。

⑥曾经有一出戏代表了一个时代对于风的强硬态度,它的名字叫《战台风》。当几百万知青躬耕于大野,对于自然的力量不寒而栗时,这出戏的出现,增添了人们不自量力的胆魄,面对台风、暴雨、山洪、泥石流,以血肉之躯去抵挡的人不在少数。

⑦然而,自然界永远是一幅广阔高迥的相貌,深含超人的力量。人类无法拒绝风,无法改变风的走向,人的自尊自信受到了风的挑战。人在长大,除了肉身的伸张之外,更重要的标志是人的思维摆脱了愚昧,走上了正常的轨迹。我注意到,面对破坏力强大的风,人们已经从迎战转为躲避,避其锋芒、尖锐,不再侈谈人定胜天这类空话。渔船靠港、学校放假,举村迁移,这些都是认识上的觉醒和进步——没有什么比人的生命更加紧要,人在大自然自然面前低头,并不是人类的羞耻。

⑧在风中,我们长大。                                                    (选自《十月》,有删节)

小题1:文章第③节描写了家庭生活温馨的一幕,请简说其表达特色。(4分)

小题2:文章第⑥节写《战台风》这出戏有何作用?(6分)

小题3:文章题目“在风中长大”中“长大”的内涵有哪些?(4分)

小题4:本文蕴含着作者丰富的情感,请予以探究。(6分)

阅读理解

The economic growth that many nations in Asia and increasingly Africa have experienced over the past couple of decades has transformed hundreds of millions of lives – almost entirely for the better. But there’s a byproduct to that growth, one that’s evident – or sometimes less than evident – in the smoggy, smelly skies above cities like Beijing, New Delhi and Jakarta. Thanks to new cars and power plants, air pollution is bad and getting worse in much of the world, and it’s taking a major toll (伤亡人数,代价) on global health.

How big? According to a new analysis published in the Lancet, more than 3.2 million people suffered deaths from air pollution in 2010, the largest number on record. That’s up from 800,000 in 2000. And it’s a regional problem: 65% of those deaths occurred in Asia, where the air is choked by diesel soot (内燃发动机烟雾) from cars and trucks, as well as the song from power plants and the dust from endless urban construction. In East Asia and China, 1.2 million people died, as well as another 712,000 in South Asia, including India. For the first time ever, air pollution is on the world’s top – 10 list of killers, and it’s moving up the ranks faster than any other factor.

So how can air pollution be so damaging? It is the very finest soot – so small that it roots deep within the lungs and then enters the bloodstream – that contributes to most of the public – health toll of air pollution including death. Diesel soot, which can also cause cancer, is a major problem because it is concentrated in cities along transportation zones affecting overpopulated areas. It is thought to contribute to half the deaths from air pollution in urban centers. Fro example, 1 in 6 people in the U.S. live near a diesel – pollution hot spot like a rail yard, port terminal or freeway.

We also know that air pollution may be linked to other non – deadly diseases. Fortunately in the U.S. and other developed nations, urban air is for the most part cleaner than it was 30 or 40 years ago, thanks to regulations and new technologies like the catalytic converters (催化式排气净化器) that reduce automobile emissions. Govemments are also pushing to make air cleaner – see the White House’s move last week to further tighten soot standards. It’s not perfect, but we’ve had much more success dealing with air pollution than climate change.

Will developing nations like India eventually catch up? Hopefully – though the problem may get worse before it gets better. The good news is that it doesn’t take a major technological advance to improve urban air. Switching from diesel fuel to unleaded (无铅燃油) helps, as do newer and cleaner cars which are less likely to send out pollutants. Power plants – even ones that burn mineral fuels like coal – can be fitted with pollution – control equipment that, at a price, will greatly reduce smog and other pollutants.

But the best solutions may involve urban design. In the Guardian, John Vidal notes that Delhi now has 200 cars per 1,000 people, far more than much richer Asian cities like Hong Kong and Singapore. Developing cities will almost certainly see an increase in car ownership as residents become wealthier – and that doesn’t have to mean deadly air pollution. Higher incomes should also lead to tougher environmental regulations, which is exactly what happened in the West. We can only hope it happens before the death toll from bad air gets even higher.

小题1:What tends to give rise to the highest death toll according to the passage?

A.The lack of tight environmental protection standards.

B.The increasing numbers of the diesel cars and trucks.

C.The frighteningly high death rate from deadly cancer.

D.The world’s serious air pollution such as soot and dust.小题2:The “byproduct” (Paragraph 1) most probably refers to     .

A.consequence

B.solution

C.reform

D.design小题3:The basic reason why so many people die from air pollution is that       .

A.the diesel soot is too small to be seen

B.the diesel soot is much too poisonous to breathe

C.the diesel soot roots in lungs and gets into blood

D.the diesel soot can also contribute to deadly cancer小题4:According to the passage, the writer actually wants to convince the readers that        .

A.the global economic growth is mainly to blame for air pollution and climate change

B.the developing countries are repeating the same mistakes as the developed ones made

C.the ecological situation and air pollution in India are becoming worse and worse

D.the unbeatable air is increasingly becoming a major killer throughout the world小题5:By describing urban design as “the best solution” in the last paragraph, the writer means that        .

A.the making of tougher environmental regulations alone is of little use

B.more sever regulations should be made to handle air pollution

C.the urban construction in western developed countries is the best choice

D.the pace of development has to be slowed down to reduce air pollution