问题 选择题

如图所示,一玻璃管中注满清水,水中放一软木塞R(软木塞的直径略小于玻璃管的直径,轻重大小适宜,使它在水中能匀速上浮)。将玻璃管的开口端用胶塞塞紧(图甲),现将玻璃管倒置(图乙),在软木塞上升的同时,将玻璃管水平向右加速移动,观察木塞的运动,将会看到它斜向右上方运动。经过一段时间,玻璃管移至图丙中虚线所示位置,软木塞恰好运动到玻璃管的顶端,在下面四个图中,能正确反映软木塞运动轨迹的是  (      )

 

答案

答案:C

题目分析:软木塞参加了两个分运动,竖直方向在管中以匀速上浮,水平方向水平向右匀加速直线移动,速度不断变大,将合成,如图

由于曲线运动的速度沿着曲线上该点的切线方向,又由于不变,不断变大,故θ不断变小,即切线方向与水平方向的夹角不断变小,故ABD均错误,C正确;

故选C.

点评:本题关键由分运动速度合成出合速度后,得到合速度方向的变化规律,再结合轨迹讨论即可.

单项选择题
单项选择题

Passage Two

During the last thirty years, the international economy has experienced a basic change. Improvements in the Internet and other communications have had important effects on world markets. Faster methods of transport from one place to another have made the world seem smaller to the businessmen. As a result, the world no longer. consists of a number of separate economies under the control of different nations. Instead, the nations have been integrated into a single economy, and the integration is becoming more and more complete every year. For the first time in history, we can truly speak of a global economy.
The most dramatic example of this integration was the oil crisis of 1973 and 1974. It came as a great surprise to the public in the industrial nations to discover that they depended so heavily on imported oil. However, the best evidence for the growing integration is the rapid expansion in the volume of world trade. It went up by about 7% a year during the decade from 1990 to 2000, and in several quasi-industrial (半工业的) countries the growth was even more rapid. As a result, some imported products have become as common as domestic commodity. Some of them are too common for the public to any more realize they are foreign.
Production has also become international, which is manifested by the large corporations stepping across national borders and established branches and subsidiaries in several different countries. As an example, U.S. companies are building automobiles in Canada, Germany, Britain and Japan. In some cases, components of an automobile are produced abroad and shipped to the United States, where it is then finished with the imported parts. When even the United States has the largest number of such corporations, it is not the only. Other multinationals, for instance, are based in Japan, France, Germany, the UK and Italy.
Labor, too, is much more mobile than in the past. Both skilled and unskilled workers can now readily migrate from one country to another. In Europe, for example, there are large numbers of Turkish workers employed in the Germany Economy. Doctors, lawyers, and other professionals are also finding it easier to work in foreign countries. The labor market has become international, and the number of expatriate workers is continuing to grow.

The passage is mainly about ______ .

A.the dependence of the industrial countries on imported oil

B.the fundamental change in world economy

C.the expansion of international corporations

D.the Internet and world economy