将一根长L=2.75m的链条自由下垂悬挂在墙上，放开后让链条作自由落体运动，悬点O正下方有一个高h为1.8m圆筒AB，圆筒顶端A距悬点O为H=3.2m；（g=10m/s2），求
（1）从开始到链条下端刚到A点时经历的时间t1．
（2）链条通过圆筒AB点经历的时间t．
（1）链条做自由落体运动，根据几何关系求解出从开始到链条下端刚到A点过程的位移为H-L，根据位移时间关系公式，有
H-L=
g1 2 t 21
代入数据解得t1=0.3s
（2）链条做自由落体运动，根据几何关系求解出从开始到链条上端刚到B点过程的位移为L+h+H，然后根据位移时间关系，有
L+H+h=
g1 2 t 22
代入数据解得t2=1s
链条通过圆筒AB点经历的时间
t=t2-t1=1-0.3=0.7s
答：（1）从开始到链条下端刚到A点时经历的时间t1为0.3s；
（2）链条通过圆筒AB点经历的时间t为0.7s．
Read the following passage. Complete the diagram by using the information from the passage. Write NO MORE THAN THREE WORDS for each answer. Most scientists are now certain that global warming is taking place. Gases such as carbon dioxide produced by burning of coal, oil, wood, together with industrial pollution, are creating a warm blanket around the earth. This blanket is trapping heat in the atmosphere and so raising the temperature of the earth. The evidence for global warming can now be seen in the world’s changing climate statistics. In Europe, eight of the last ten years have seen record high temperature. For northern Europe, this has generally been a change for the better. Gardens can now even grow tropical plants in England, though London may never see a “White Christmas” again. On the other hand, the countries around the Mediterranean Sea, and those south of the Sahara desert are receiving even less rain than before. In sub-Saharan Africa the crops are drying out in the fields and people are dying of starvation. In the Americans, the climate is becoming more extreme—the summers are getting hotter and the storms are becoming more violent. In 1999 the southern United States was struck by a series of destructive hurricanes, while the end of 1999 saw the worst floods ever in Venezuela. Meteorologists expect such trends to continue, and indeed to worsen, if global warming cannot be stopped. In addition to worrying about rising global temperatures and more extreme weather conditions, scientists are closely monitoring sea levels around the world. These are slowly rising, as the northern and southern polar ice-caps start to melt. This will have serious consequences for low-lying countries near the sea, such as the coral islands in the Pacific, and Bangladesh where the River Ganges already floods the delta（三角洲） every year. Already parts of these places are disappearing under the rising tides. Title: 71._______________.      
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