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2012年12月28日，武汉地铁2号线正式通车．已知相距4000m的甲乙两站之间的线路为直线，在调试运行时，列车出站加速和进站减速的加速度大小均为1m/s2，中途保持匀速运行．列车从甲站由静止出发到乙站停止正好用时4min．某次调试时，列车从甲站出发比原定的出发时间晚了20s，但仍保持与原来相同的加速度出站和进站，要使列车仍然准点到达乙站，求此次列车出站加速的时间应为多少？

答案

设列车本次出站应加速的时间t0，列车从甲站到乙站实际用时t，列车加速出站过程的位移为

x1=

1
2
at02，

同理，列车进站过程的位移为 x2=

1
2
at02．

列车匀速运行的速度为 v=at0．

列车匀速前进过程的位移为 x0=v（t-2t0），

又 x1+x0+x2=x

代入数据联立解得 t0=20s．

答：此次列车出站加速的时间应为20s．

单项选择题

Humans are social animals. They live in groups all over the world. As these groups of people live apart from other groups, over the years and centuries they develop their own habits and ideas, which form different cultures. One important particular side of every culture is how its people deal with time.
Time is not very important in non-industrial societies. The Nuer people of East Africa, for example, do not even have a word TIME that is in agreement with the abstract thing we call time. The daily lives of the people of such non-industrial societies are likely to be patterned around their physical needs and natural events rather than around a time schedule based on the clock. They cook and eat when they are hungry and sleep when the sun goes down. They plant crops during the growing seasons and harvest them when the crops are ripe. They measure time not by a clock or calendar, but by saying that an event takes place before or after some other events. Frequently such a society measures days in terms of "sleeps" or longer periods in terms of "moons". Some cultures, such as the Eskimos of Greenland measure seasons according to the migration of certain animals.
Some cultures which do not have a written language or keep written records have developed interesting ways of "telling time". For example, when several Australian aborigines want to plan an event for a future time, one of them places a stone on a cliff or in a tree. Each day the angle of the sun changes slightly. In a few days, the rays of the sun strike the stone in a certain way. When this happens, the people see that the agreed-upon time has arrived and the event can take place.
In contrast, exactly correct measurement of time is very important in modern, industrialized societies. This is because industrialized societies require the helpful efforts of many people in order to work. For a factory to work efficiently (well, quickly and without waste), for example, all of the workers must work at the same time. Therefore, they must know what time to start work in the morning and what time they may go home in the afternoon. Passengers must know the exact time that an

By saying "Humans are social animals", the author means ______.

A. they live all over the world
B. they are different from other animals
C. they live in one place, district or country, considered as a whole
D. they are divided into many groups
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单项选择题

下列情形中，宗地的界址信息未发生变更的是（）。

A.土地整理后的宗地重划

B.宗地所属地区的区划的变动

C.土地权属界址调整

D.新增出让国有土地
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