问题 问答题

如图所示,某人乘雪橇从A点以2.0m/s的速度飞出,落到B点时的速度为12.0m/s,接着沿水平路面滑至C点停止.人与雪橇的总质量为70kg.(人在空中运动时所受阻力不计,人落地时能量损失也不计).试求:

(1)A点距BC的高度h为多少?

(2)设人与雪橇在BC段所受阻力为自身重力的0.2倍,求BC距离S多大 (g=10m/s2

答案

(1)从A到B的过程中只有重力做功,机械能守恒,根据机械能守恒定律得:

mgh=

1
2
mvB2-
1
2
mvB2

即70×10×h=

1
2
×70×(12)2_ 
1
2
×70×(2)2

解得:h=7m

(2)人与雪橇在BC段所受阻力f=0.2mg=0.2×70×10=140N

1
2
mv2=fs

解得:s=

1
2
×70×12×12
140
m=36m

答:(1)A点距BC的高度h为7m;

(2)BC距离S为36m.

填空题
单项选择题

The question of whether war is inevitable is one, which has concerned many of the world’s great writers. Before considering this question, it will be useful to introduce some related concepts. Conflict, defined as opposition among social entities directed against one another, is distinguished from competition, defined as opposition among social entities independently striving for something, which is in inadequate aupply. Competitors may not be aware of one another, while the parties to a conflict are. Conflict and competition are both categories of opposition, which has been defined as a process by which social entities function is the disservice of one another.

Opposition is thus contrasted with cooperation, the process by which social entities function in the service of one another. These definitions are necessary because it is important to emphasize that competition between individuals or groups in inevitable in a world of limited resources, but conflict is not. Conflict, nevertheless, is very likely to occur, and is probably an essential and desirable element of human societies.

Many authors have argued for the inevitability of war from the premise that in the struggle for existence among animal species, 0nly the fittest survive. In general, however, this struggle in nature is competition, not conflict. Social animals, such as monkeys and cattle, fight to win or maintain leadership of the group. The struggle for existence occurs not in such fights, but in the competition for limited feeding areas and for occupancy of areas free from meat-eating animals. Those who fail in this competition starve to death or become victims to other species. This struggle for existence does not resemble human war, but rather the competition of individuals for jobs, markets, and materials. The essence of the struggle is the competition for the necessities of life that are insufficient to satisfy all.

Among nations there is competition in developing resources, trades, skills, and a satisfactory way of life. The successful nations grow and prosper; the unsuccessful decline. While it is true that this competition may induce efforts to expand territory at the expense of others, and thus lead to conflict, it cannot be said that war-like conflict among nations is inevitable, although competition is.

The author indicates in the passage that conflict()

A. is an inevitable struggle resulting from competition

B. reflects the struggle among social animals

C. is an opposition among individual social entities

D. can be avoided