问题 选择题

某滑沙场有两个坡度不同的滑道AB和AB'(均可看作斜面),甲、乙两名旅游者分别乘两个完全相同的滑沙撬从A点由静止开始分别沿AB和AB'滑下,最后都停在水平沙面BC上,如图所示.设滑沙撬和沙面间的动摩擦因数处处相同,斜面与水平面连接处均可认为是圆滑的,滑沙者保持一定姿势坐在滑沙撬上不动.则下列说法中正确的是(  )

A.甲滑行的总路程一定大于乙滑行的总路程

B.甲在B点的动能一定大于乙在B'点的动能

C.甲在B点的速率一定等于乙在B'点的速率

D.甲全部滑行的水平位移一定大于乙全部滑行的水平位移

答案

A:设甲在水平面上的位移为S,斜面长度为L,斜面所对应的水平位移为X,对甲从斜面下滑至停止这段由动能定理得:

mgh-μmgcosθL-μmgS=0

即:

mgh-μmgX-μmgS=0

整理得:

X+S=

h
μ

而X+S即为物体甲运动的水平位移,可见它是一个与倾角无关的量,即对甲乙来说,其水平位移是相等的,也即甲和乙是停在水平面同一点上.

由数学三角关系可以知甲的总路程大于乙的总路程.故A正确.

B:设甲下滑到斜面末端的速度为v,对甲从斜面下滑至末端由动能定理得:

mgh-μmg

h
sinθ
=
1
2
mv2

可知物体到斜面末端的动能与夹角及质量有关,由于不知道甲乙质量关系,所以无法判断谁大,故B错误.

C:从B的动能定理表达式可以整理得到:v2=2gh-

2μgh
sinθ
,可知速度只与夹角有关,故甲在斜面末端的速度大于乙在斜面末端的速度.

D:由A的计算知道甲乙水平位移相等.故D错误.

故选A

阅读理解

Microsoft founder Bill Gates has recovered his spot at the top of the US money heap, taking the place of investor Warren Buffett as America's richest person, Forbes magazine's latest list reveals.

With 57 billion dollars net worth Gates again leads the list of 400 richest individuals in the world's wealthiest country. He displaced Buffett who briefly held the position this year but who has seen his Berkshire Hathaway investment group's shares slip 15 percent since February and is now worth 50 billion.

According to Forbes, whose list was published late Wednesday, the golden 400 have 1.3 billion dollars net worth or more. However, their combined net worth rose only 30 billion dollars, or two percent, to 1.57 trillion dollars.

Forbes said that rising oil and dizzy art prices fuelled the entry of 31 new members into the ultra-rich club and the return of eight previous members.

A notable arrival was Mark Zuckerberg, 24, founder of the social networking site Facebook(脸谱网). Forbes estimates his worth at 1.5 billion dollars.

Meanwhile, turmoil(动荡)on the stock and housing markets saw 33 others drop off the list, including the former head of the troubled insurance giant AIG, Maurice Greenberg, and a former head of the online auction site eBay, Margaret Whitman.

Biggest gainers were led by New York Mayor Michael Bloomberg who took eighth place with 20 billion dollars worth after a transaction(交易)put a new value on his Bloomberg media and financial data network.

The biggest loser was casino(赌场,娱乐场)tycoon Sheldon Adelson, whose fortune fell 13 billion dollars over 12 months -- the equivalent to 1.5 million dollars an hour -- although he still has 15 billion dollars and occupies 15th place.

About two thirds of the list are self-made billionaires and just over 10 percent are women, led by television star Oprah Winfrey whose fortune rose 200 million dollars to 2.7 billion dollars.

小题1: According to Forbes, Warren Buffet is worth _____.

A.57 billion dollars

B.50 billion dollars

C.1.5 billion dollars

D.20 billion dollars小题2:31 new members entered the ultra-rich club as a result of _____.

A.turmoil on the stock and housing market

B.media and financial data network

C.rising oil and dizzy art prices

D.investment小题3:The purpose of the author using the television star Oprah Winfrey as an example is to _____.

A.tell the readers that television stars make money easily

B.prove that a millionaire can become a billionaire

C.women can also be billionaires

D.tell the readers that most of the billionaires are self-made小题4:It can be inferred from the story that _____.

A.one earns much and also loses much

B.stability of markets has much effect upon people’s wealth

C.the computer industry makes more billionaires

D.young people can also be billionaires小题5:What is the best title of the passage?

A.Bill Gates Leading World’s Richest People Again

B.What Made Billionaires

C.Self-Made Billionaires

D.Biggest Gainer and Biggest Loser

单项选择题