问题 单项选择题

下图所示,是叫做蹦极的游戏,游戏者将一根有弹性的绳子一端系在身上,另一端固定在高处,从高处跳下。图中a点是弹性绳自然下垂时绳下端到达的位置,c点是游戏者所到达的最低点。对于游戏者离开跳台至最低点的过程,下列说法正确的是()

A.游戏者的动能一直在增加

B.游戏者减少的重力势能全部转化为动能

C.游戏者通过a点之后,绳子具有弹性势能

D.游戏者到达c点时,他的动能最大

答案

参考答案:C

解析:

游戏者离开跳台到达a点前,弹性绳不受拉力。游戏者的重力势能随其下降而减小,动能增大,重力势能转化为动能。在a点时,动能最大,速度最大;通过a点后,由于受弹性绳向上的拉力,使其速度减小,动能减小,同时弹性绳受游戏者拉力而伸长,将游戏者的一部分重力势能转化为绳的弹性势能;当游戏者到达c点时动能为零,全部转化为弹性势能。

句型转换
阅读理解

Suppose that we lined up our roughly 14 million United States businesses in order of size, starting with the smallest, along an imaginary road from San Francisco to New York. There will be 4,500 businesses to the mile, or a little less than one per foot. Suppose further that we planted a flag each business. The height of the flag pole represents the yearly volume of sales (销量) , each $ 10, 000 in sales in shown by one foot of pole.

The line of flagpoles is a very interesting sight. From San Francisco to about Reno, it is almost unnoticeable, a row of poles about a foot high. From Reno eastward the poles increase in height until, near Columbus, Ohio - about four - fifths of the way across the nation - flags fly about 10 feet in the air.

But as we approach the eastern terminus (终点) , the poles suddenly begin to mount (升高) . There are about 300, 000 firms in the country with sales over $ 500, 000. These firms take up the last 75 miles of the 3,000 - mile road. There are 200, 000 firms with sales over $ 1 million. They take up the last 50 miles. Then there are 1, 000 firms with sales of $ 50, 000, 000 or more. They take up the last quarter of a mile before the city limits, flags flying at cloud height, 5 , 000 feet up .

At the very gate of New York, on the last 100 feet of the last mile , we find the 100 largest industrial firms . They have sales of at least $ 1.5 billion, so that their flags are already miles high. Along the last 10 feet of road , there are 10 largest companies . Their sales are roughly $ 10 billion and up : their flags fly 190 miles in the air , almost in the stratosphere (平流层) .

小题1:What is the author’s main purpose in this passage?

A.To tell the reason why the largest firms are in New York.

B.To show the geographical distribution (分布) of the United States businesses.

C.To provide a general idea of the size of businesses in the United States.

D.To tell us how the United States businesses are arranged.小题2:What’s the correct order of the following places from east to west?

A.San Francisco, Reno, New York, Columbus.

B.San Francisco, Reno, Columbus, New York.

C.New York, Columbus, Reno, San Francisco

D.New York, Reno, Columbus, San Francisco小题3:Among the four parts of the imaginary road , which part , considering all the firms in it , has the largest total volume of sales ?

A.The last 75 miles.

B.The last 50 miles.

C.The last 100 feet of the last mile.

D.The last quarter of a mile小题4:Which of the following diagrams (图表) shows the right order of size of the US businesses , according to the text ?

S =" San" Francisco  R =" Reno"  C =" Columbus"  N =" New" York  H =" height"  F = feet