问题 计算题

如图所示,光滑圆轨道ABC,其中AB部分水平, BC部分是处于竖直平面内的半径为R的半圆管,圆管内截 面的半径r<R,有一质量为m、半径略小于r的小球以水平速度v0从A点射入圆管.问:

(1)若要小球能从C端飞出,初速度v0应满足什么条件?

(2)在小球从C端飞出的瞬间,对管壁作用力有哪几种可能情况?初速度v0各应满足什么条件?

答案

解:

(1)小球从A端入射后,若刚好到达C点,则vC=0,小球A→B→C的过程中只存重力做功,机械能守恒.由机械能守恒定律得: ,即故若小球能从C端飞出,入射速度应大于

(2)小球从C端飞出时,对管壁的作用力有以下三种可能情况.

a.恰好对管壁无压力,此时由牛顿第二定律知

                   ①

又A→B→C过程中小球机械能守恒,

由①②解得

b.对下管壁有压力,此时的入射速度

c.对上管壁有压力,此时的入射速度

问答题 简答题
单项选择题

Several years ago, at the height of the dotcom boom, it was widely assumed that a publishing revolution, in which the printed word would be supplanted by the computer screen, was just around the corner. It wasn’t: for many, there is still little to match the joy of reading a printed book and settling down for one hour. But recently some big technology companies, including Google, Amazon, Microsoft and Yahoo, contend that the dream of bringing books online is still very much alive.

The digitizing of thousands of volumes of print is not without controversy. On Thursday, Google, the world’ s most popular search engine, posted a first installment of books on Google Print. This collaborative effort between Google and several world’s leading research libraries aims to make books available to be searched and read online free of charge. Although the books included so far are not covered by copyright, the plan has attracted the rage of publishers.

Five large book firms are suing Google for violating copyright on material that it has scanned and, although out of print, is still protected by law. Google has said that it will only publish short extracts from material under copyright unless given express permission to publish more, but publishers are unconvinced. Ironically, many publishers are collaborating with Google Print Publisher, which aims to give readers an online taste of books that are commercially available. The searchable collection of extracts and book information is intended to tempt readers to buy the complete books online or in print form.

Amazon, the world’s largest online retailer, has made plans to enter the mass e-book market by selling a vast array of goods. Given that Google should impinge upon its central territory, Amazon revealed that it would introduce two new services. Amazon Pages will allow customers to search for key terms in selected books and then buy and read online whatever part they wish. Amazon Upgrade will give customers online access to books they have already purchased as hard copies. Customers are likely to have to pay five cents a page, with the bulk going to the publisher.

Microsoft has also joined the online-book trend. In October, the software giant said it would spend around $ 200 million to digitize texts, starting with I50,000 that are in the public domain, to avoid legal problems. It will do so in collaboration with the Open Content Alliance. And on Thursday, coincidentally the same day as Google and Amazon announced their initiatives, Microsoft released details of a deal with the British Library, the country’s main reference library, to digitize some 25 million pages ; these will be made available through MSN Book Search, which will be launched next year.

What is the text talking about()

A. The goods of Google

B. The differences between books printed and books online

C. Some big technology companies bring plans on the e-books

D. The publishers don’t agree with the e-books plan