问题 问答题

汽车刹车前的速度为20m/s,刹车获得的加速度大小为4m/s2,则:

(1)汽车开始刹车后10s内滑行的距离是多少?

(2)从开始刹车到汽车通过42m的位移所经历的时间是多长?

(3)汽车停止运动前3s内向前滑行通过的距离是多少?

答案

(1)根据速度时间关系公式,汽车运动的总时间为:t=

0-v0
a
=
0-20
-4
=5s即汽车刹车后5s已经停止前进,故汽车在10s内滑行的距离等于5s内滑行的距离,故有

x=v0t+

1
2
at2=20×5+
1
2
×(-4)×25=50m

(2)设从开始刹车到汽车通过42m的位移所经历的时间为t,由x=v0t+

1
2
at2

42=20t+

1
2
×(-4)×t2

解得

t1=3s或t2=7s(舍去)

(3)汽车刹车后前2s内位移为x1=20×2+

1
2
×(-4)×4=32m

故汽车停止运动前3s内的位移为x2=x-x1=50m-32m=18m

答:(1)汽车开始刹车后10s内滑行的距离是50m;

(2)从开始刹车到汽车通过42m的位移所经历的时间是3s;

(3)汽车停止运动前3s内向前滑行通过的距离是18m.

单项选择题

Anthropology is the study of human beings as creatures of society. It (1) its attention upon those. physical characteristics and industrial techniques, those conventions and values, which (2) one community from all others that belong to a different tradition.

The distinguishing mark of anthropology among the social sciences is that it includes for serious study other societies (3) our own. For its purposes any social (4) of mating and reproduction is as significant as our own. To the anthropologist our customs and those of a New Guinea tribe are two possible social schemes for (5) a common problem, and in so far as he remains an anthropologist he is (6) to avoid any weighting of one (7) the other. He is interested in human behavior, not as it is shaped by one tradition, our own, but as it has been shaped by any tradition (8) He is interested in a wide (9) of custom that is found in various cultures, and his object is to understand the way in which these cultures change and (10) , the different forms through which they express themselves and the (11) in which the customs of any peoples function in the lives of the (12) .

Now custom has not been commonly regarded as a (13) of any great moment. The inner workings of our own brains we feel to be uniquely (14) of investigation, but custom, we have a way of thinking, is behavior at its most commonplace. (15) , it is the other way round. Traditional custom is a mass of detailed behavior more astonishing than (16) any one person can ever evolve in individual actions. Yet that is a rather (17) aspect of the matter. The fact (18) first rate importance is the predominant role that custom (19) in experience and belief, and the very great varieties it may (20)

19()

A.functions

B.plays

C.makes

D.leads

名词解释