问题 问答题

如图所示,滑块在恒定的水平外力F=2mg的作用下,从水平轨道上的A点由静止出发到B点时撤去外力,又沿竖直面内的光滑半圆形轨道运动,且恰好通过轨道最高点C,滑块离开半圆形轨道后又刚好落到原出发点A,求AB段与滑块间的动摩擦因数.

答案

设圆周的半径为R,则在C点:mg=m

v2c
R
…①

滑块离开C点,做平抛运动,则

2R=

1
2
gt2… ②

LAB=vct…③

滑块由B到C过程,由机械能守恒定律得:

1
2
mv2B
=
1
2
mv2C
+2mgR…④

滑块由A到B运动过程,由动能定理得:

(F-μmg)LAB=

1
2
mv2B
…⑤

由①②③④⑤式联立得到:

μ=0.75

答:AB段与滑块间的动摩擦因数为0.75

阅读理解

  Napoga  is a 12—year---old  girl in Ghana, Africa. It is hard for her family to get clean water . Every morning , she leaves home at half past five to get clean water for her family in a village far away . It takes her six hours to get enough clean water for daily cooking and drinking. She has no time to go to school or to play with her friends. Millions of people in the world are like Napoga . They can’t get enough clean water to keep healthy.

Earth Day is April 22. But on all other days, we must also remember it. The water we use is the most important natural resource(自然资源)on the earth.

Water covers 70 perent of the earth’s surface (表面). But most of that is sea water. We can’t use it for very many things.  Fresh water covers only one percent of the earth’s surface.

You probably feel lucky that your life isn’t as hard as Napoga’s . But that doesn’t mean you don’t have to worry about water. We all face serious water problems. One of them is water pollution. All kinds of things from cars, factories, farms and homes make our rivers , lakes , and oceans dirty. Polluted water is very bad for people to drink. And dirty water is bad for fish , too. Now, thirty-four percent of all kinds of fish are dying out.

How do cars and factories make our water dirty? First, they pollute the air. Then , when it rains, the rain water comes down and makes our drinking water dirty. Dirty rain, called acid rain(酸雨), is also bad for plants ,animals and buildings.

Scientists say that in 30 years, more than half of the people in the world won’t have enough clean water. We have to learn how to save more water for ourselves  and our children. Here is some advice for saving water:

● Turn off the water while you brush your teeth. You can save as much as 450liters(升)each month.

● Leaky taps (漏水的龙头)waste a lot of water. Fix them right away.

●You can easily cut your 10—minute showers in half---and you’ll be just as clean.

●When you wash dishes, don’t let the water run.

●Only wash clothes when you have a lot to wash. If your washing machine isn’t full , you are wasting water!

根据材料内容,选择最佳答案,并将其标号填入题前括号内。

小题1:Every morning Napoga went to get clean water _________.

A.before she went to school

B.after she finished school

C.without going to school

D.after she played with her friends小题2:On the earth, _______________.

sea water is  more widely used than fresh water

B.there is very little water for our living

C.there is 71 percent of fresh water

D.we don’t need to worry about water

小题3:Acid rain____________________________.

A.is only bad for living things.

B.can be produced in factories

C.doesn’t do harm to people , but it’s bad for plants, animals and buildings.

D.doesn’t just do harm to human beings but to animals, plants and buildings.小题4:Which of the following wastes water?

A.We need ten minutes if we want a clean shower.

B.We should put in enough clothes when we wash them in washing machines.

C.You can keep yourself clean by having a shower for five minutes.

D.Don’t let the water run when we brush our teeth and wash dishes.小题5:What’s the main idea of the passage?____________.

A.Something must be done to prevent water from being polluted.

B.Clean water means much to human beings and living things around us.

C.It’s time to take care of the water we use.

D.Water is the most important natural resource on the earth.

单项选择题

"You are not here to tell me what to do. You are here to tell me why I have done what I have already decided to do," Montagu Norman, the Bank of England’s longest-serving governor (1920-1944), is reputed to have once told his economic adviser. Today, thankfully, central banks aim to be more transparent in their decision making, as well as more rational. But achieving either of these things is not always easy. With the most laudable of intentions, the Federal Reserve, America’s central bank, may be about to take a step that could backfire.

Unlike the Fed, many other central banks have long declared explicit inflation targets and then set interest rates to try to meet these. Some economists have argued that the Fed should do the same. With Alan Greenspan, the Fed’s much-respected chairman, due to retire next year—after a mere 18 years in the job—some Fed officials want to adopt a target, presumably to maintain the central bank’s credibility in the scary new post-Greenspan era. The Fed discussed such a target at its February meeting, according to minutes published this week. This sounds encouraging. However, the Fed is considering the idea just when some other central banks are beginning to question whether strict inflation targeting really works.

At present central banks focus almost exclusively on consumer-price indices. On this measure Mr. Greenspan can boast that inflation remains under control. But some central bankers now argue that the prices of assets, such as houses and shares, should also somehow be taken into account. A broad price index for America which includes house prices is currently running at 5.5%, its fastest pace since 1982. Inflation has simply taken a different form.

Should central banks also try to curb increases in such asset prices Mr. Greenspan continues to insist that monetary policy should not be used to prick asset-price bubbles. Identifying bubbles is difficult, except in retrospect, he says, and interest rates are a blunt weapon: an increase big enough to halt rising prices could trigger a recession. It is better, he says, to wait for a housing or stock market bubble to burst and then to cushion the economy by cutting interest rates—as he did in 2001-2002.

And yet the risk is not just that asset prices can go swiftly into reverse. As with traditional inflation, surging asset prices also distort price signals and so can cause a misallocation of resources—encouraging too little saving, for example, or too much investment in housing. Surging house prices may therefore argue for higher interest rates than conventional inflation would demand. In other words, strict inflation targeting—the fad of the 1990s—is too crude.

Which of the followings would be the best title for the text()

A. American Monetary Conundrums Are Readily Deciphered

B. American Central Banks Are on the Verge of Extinction

C. Conventional Inflation Target Is Best Employed in Transparent Environment

D. America’s Monetary Policy Is off Target