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问题 计算题

如图所示，ABC和DEF是在同一竖直平面内的两条光滑轨道，其中ABC的末端水平，DEF是半径为r＝0.4 m的半圆形轨道，其直径DF沿竖直方向，C、D可看成重合。现有一可视为质点的小球从轨道ABC上距C点高为H的位置由静止释放。  

(1)若要使小球经C处水平进入轨道DEF后能沿轨道运动，H至少要有多高？  

(2)若小球静止释放处离C点的高度h小于(1)中H的最小值，小球可击中与圆心等高的E点，求h。(g＝10 m/s2)

答案

解：(1)小球从ABC轨道下滑，设到达C点时速度大小为v，则：mgH＝mv2

要使小球能在竖直平面DEF内做圆周运动，在D点，有

联立以上两式并代入数据得：H≥0.2 m

(2)若h＜H，小球过C点只能做平抛运动，设小球经C点时速度大小为vx，小球能击中E点，则有

由以上三式解得：h＝0.1 m

单项选择题

You can tell the age of a tree by counting its ring. But these records of a tree’s life really say a lot more. Scientists are using tree ring to learn what’s been happening on the sun’s surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, it adds a layer to its trunk, taking up chemical elements from the air. By looking at elements in the rings from a certain years, scientists can tell what elements were in the air that year. Dr. Stevenson is analyzing one element, carbon-14, in ring from both living and dead tree. Some ring go back almost ten thousand year to the end of the Ice Age. When Stevenson followed the carbon-14 levels changed with the intensity of solar burning. You see, the sun has cycles. Sometimes it bums fiercely, and at other times it is relatively calm. During the sun’s violent periods, it throw off charged particles in fast moving streams called "solar winds". The particularly interfere with the formation of carbon-14 on earth. When there is more solar wind activity, less carbon-14 is produced. Ten thousand years of tree ring show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that solar wind activity must follow the same cycle.

The purpose of the scientists in studying tree ring is to ().

A. examine the chemical elements in Ice Age

B. look into the pattern of solar wind activity

C. analyze the composition of different trees

D. find out the origin of carbon-14 on earth
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单项选择题

基因治疗中，可用做活体直接转移的基因载体是（）

A.大肠杆菌

B.酵母菌

C.腺病毒

D.沙眼衣原体

E.白细胞
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