问题 选择题

如图所示,一质量为M的楔形木块放在水平桌面上,它的顶角为90°,两底角为α和β,a、b为两个位于斜面上质量均为m的小木块。已知所有接触面都是光滑的。现发现a、b沿斜面下滑,而楔形木块静止不动,这时楔形木块对水平桌面的压力等于

A.Mg+2mg

B.Mg+mg

C.Mg+mg(sinα+sinβ)

D.Mg+mg(cosα+cosβ)

答案

答案:B

题目分析:对木块a受力分析,如图,受重力和支持力

由几何关系,得到  N1=mgcosα

故物体a对斜面体的压力为N1′="mgcosα" ①

同理,物体b对斜面体的压力为N2′="mgcosβ" ②

对斜面体受力分析,如图,假设摩擦力向左

根据共点力平衡条件,得到f+N2′cosα-N1′cosβ="0" ③

F-Mg-N1′sinβ-N2′sinβ="0" ④

根据题意α+β="90°" ⑤

由①~⑤式解得F=Mg+mg

根据牛顿第三定律,斜面体对地的压力等于Mg+mg,B正确。

阅读理解

The popular college rankings focus primarily on prestige as measured by the SAT scores of incoming students and how many applicants are turned away. An initiative(措施)started last fall by the Obama administration could help families go beyond these limited, and far too easily exploited, indexes to learn quickly and easily how a college is compared with its competitors nationally on important criteria like graduation rates, what a degree actually costs and how much debt a student can expect to run up by graduation day.

If the federal government makes it legally necessary to disclose this information in a clear and consistent(一贯的)way, as it should, families will be better able to make informed college choices. And this will help put pressure on colleges that perform poorly to improve.

Critics may regard this initiative as an example of government overreach. But given that the federal government spends nearly $190 billion a year on higher education aid to students, it has a legitimate interest in making sure that the money flows to the schools that best meet their responsibilities to families and students.

Congress has taken some steps to require greater transparency(透明)from colleges. The 1990 Student Right to Know Act, for example, required colleges and universities that receive federal aid to disclose graduation rates. And the 2008 Higher Education Opportunity Act required schools to offer a way for consumers to determine actual costs after student aid is taken into account.

But many colleges have done a poor job of obeying federal disclosure rules, and much of the available information is not in one place. The administration’s new efforts would enforce reporting requirements and provide some new tools.

President Obama wants to expand campus-based aid to about $10 billion from the current $2.7 billion. He has proposed moving money away from colleges that fail to control tuition increases or provide good value to others that do a better job. That is a worthy idea in principle, but he will need strong data-based evidence to determine how colleges are doing.

The transparency initiatives are a good place to start and should be embraced by both parties in Congress. If students and families, facing higher tuition and rising debt, are to make sound choices, they need more and better information.

小题1:Why did the Obama administration start the initiative last fall?

A.To require colleges to make their graduation rates known to the public.

B.To help colleges perform better in the future.

C.To help parents and students make better choices of colleges.

D.To put more pressure on colleges that are not doing well enough.小题2:Which of the following statements is TRUE according to the passage?

A.Critics agree that the government should require colleges to obey the disclosure rules.

B.Congress has approved and made the transparency initiatives a law.

C.SAT scores alone do not determine one’s choice of college.

D.Students and families find it very difficult to choose their ideal colleges now.小题3:A most suitable title for the passage could be______.

A.Congress To Require Greater Transparency

B.What College Parents and Students Need to Know

C.What a Good College Is Measured By

D.Parents and Students Need to Make Wise Choices小题4:What attitude does the author hold towards the transparency initiative?

A.supportive

B.critical

C.indifferent

D.not stated

单项选择题