问题 选择题

如图所示,竖直放置的两个平行金属板间存在匀强电场,与两板上边缘等高处有两个质量相同的带电小球,小球A从紧靠左极板处由静止开始释放,小球B从两板正中央由静止开始释放,两小球最终都能运动到右极板上的同一位置,则从开始释放到运动至有极板的过程中,下列判断正确的是

A.运动时间

B.电荷量之比

C.机械能增加量之比

D.机械能增加量之比

答案

答案:B

题目分析:小球释放后在竖直方向受到重力作用,做自由落体运动,AB下落高度相等,根据两个小球运动时间选项A错。水平方向在电场力作用下做初速度0的匀加速直线运动,加速度,水平位移同一电场,电场强度相等,运动时间相等,质量相等,据图知AB水平位移之比为2:1,所以,选项B对。机械能增加量等于电场力做功,根据可得机械能增加量之比选项CD错。

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.

Which of the following is true of Google according to the text()

A. It will invariably provide free online reading for its customers

B. It has become the world’s most popular book publisher

C. It facilitates books reading and searching to a larger extent

D. It has gained permission to publish the complete books online

判断题