问题 计算题

如图所示,在光滑水平地面上有一小车,车底板光滑且绝缘,车上左右两边分别竖直固定有金属板M,N,两板间的距离为L,M板接电源的正极,N板接电源的负极,两极板间的电场可视为匀强电场,一可视为质点的带正电小球,处在小车底板上靠近M板的位置并被锁定(球与M板不接触)。小球与小车以速度v共同向右运动。已知小球带电量为q,质量为m,车、金属板和电源的总质量为3m。某时刻突然解除对小球的锁定,小球在电场力的作用下相对小车向右运动,当小球刚要与小车的N板接触时,小车的速度恰好为零。求:

(1)两极板间匀强电场的场强E的大小;

(2)从解除锁定到小球第一次运动到小车的N板时,系统电势能的变化量。

答案

解:(1)设小球和小车的加速度分别为a1和a2,由牛顿第二定律

设从解除锁定到小球刚要与小车的N板接触所经历时间为t,由题意得

v0=a2t

小球的位移

小车的位移

又s1-s2=L

联立解得

(2)由功能关系得

单项选择题
单项选择题

When the United States and Korea(SOK) announced their new free-trade agreement last month, the news was mainly economic. The deal would give American farmers and bankers alike better access to Korean consumers and help Korean companies push more electronics, cars and textiles into the United States. Largely unreported was the political angle--the U.S.-Korea(SOK) free trade agreement comes at precisely the moment when America’s military presence on the Korean Peninsula is rapidly diminishing, anti-U.S. nationalism in Korea(SOK) is growing and China is playing an ever more important leadership role in the region. This FTA is much more significant in strategic than economic terms.

It is the same about any number of trade deals in Asia these days. While free-trade agreements have always been somewhat political, solidifying national relationships, the use of FTAs in geopolitical jockeying(竞赛) is reaching new heights in East Asia. Since 1997, the number of FTAs in the region has risen from seven to 38. Last time we saw this sort of frenzied bilateral activity was back in the 1930s. That worries some economists, who fear that all the free-trade politicking will further erode an already beleaguered global trading system, and create a snowball effect of countermeasures.

It’s no accident that the activity in the region has increased since 2004, which marked the beginning of a massive free trade agreement between China and the Association of Southeast Asian Nations. China offered countries like Laos and Cambodia an "early harvest," unilaterally opening up markets for hundreds of different kinds of agricultural products. That in turn helped smooth the way for a reduction in tension in hot spots like the disputed South China Sea territories. FTAs are becoming a key instrument for great-power diplomacy.

That worries rivals, who are rushing to find their own partners. The Japanese, for example, have always been cautious when it comes to bilateral agreements. Japanese Prime Minister Shinzo Abe recently announced a new push for more Japanese FTAs in the region. Meanwhile, the EU is trying desperately to push its way back into the region, recently announcing plans to negotiate its own deals with both South Korea and the ASEAN nations.

How will all the wheeling and dealing end Not with more efficient trading. A recent map of Asian trade deals shows an increasingly complicated "spaghetti bowl" hindering broader global efforts to liberalize trade. Such deals have a disproportionately negative effect on small and medium-sized enterprises, representing as much as 80 percent of jobs in some parts of Asia. Already, the U.S.-Korea(SOK) deal is causing grousing(不满) in Japan, which would take a hit as Korean competitors no longer have. to deal with U.S. tariffs. Still, that probably won’t turn the tide -- the most important criterion in motivating a country to seek FTAs, well ahead of economic reform, was--surprise --politics.

It can be inferred from the text that ()

A. there has never been such a large-scale bilateral activity before

B. the activity in Asia has not increased until a massive F-WA is signed

C. Japanese is not eager to sign FTA despite other countries’ activities

D. an FTA helps reduce the tension in the South China Sea territories