问题 计算题

(17分)如图所示,长L=1.2 m、质量M=3 kg的木板静止放在倾角为37°的光滑斜面上,质量m=1 kg、带电荷量q=+2.5×10-4 C的物块放在木板的上端,木板和物块间的动摩擦因数μ=0.1,所在空间加有一个方向垂直斜面向下、场强E=4.0×104 N/C的匀强电场.现对木板施加一平行于斜面向上的拉力F=10.8 N. 取g=10 m/s2,斜面足够长.求:

(1)物块经多长时间离开木板?

(2)物块离开木板时木板获得的动能.

(3)物块在木板上运动的过程中,由于摩擦而产生的内能.

答案

(1)(2)27J(3)2.16J

题目分析:(1)物块向下加速由牛顿第二定律得:

对木板有:

又有:

联立解得:

(2)物块离开木板时木板的速度为:

其动能为

(3)由于摩擦而产生德尔内能为:

单项选择题
阅读理解

Hello, Chicago. If there is anyone out there who still doubts that America is a place where all things are possible, who still wonders if the dream of our founders is alive in our time, who still questions the power of our democracy(民主),tonight is your answer.

……

I was never the likeliest candidate for this office. We didn’t start with much money or many endorsements. Our campaign was not hatched in the halls of Washington. It began in the backyards of Des Moines and the living rooms of Concord and the front porches of Charleston. It was built by working men and women who dug into what little savings they had to give $ 5 and $ 10 and $ 20 to the cause.

It grew strength from the young people who rejected the myth of their generation’s apathy(冷漠), who left their homes and their families for jobs that offered little pay and less sleep.

It drew strength from the not-so-young people who braved the bitter cold and scorching heat to knock on doors of perfect strangers, and from the millions of Americans who volunteered and organized and proved that more than two centuries later a government of the people, by the people, and for the people has not perished(消失)from the Earth.

This is your victory.

And I know you didn’t do this just to win an election. And I know you didn’t do it for me.

You did it because you understand the enormity of the task that lies ahead. For even as we celebrate tonight, we know the challenges that tomorrow will bring are the greatest of our lifetime—two wars, a planet in peril(危亡), the worst financial crisis in a century.

Even as we stand here tonight, we know there are brave Americans waking up in the deserts of Iraq and the mountains of Afghanistan to risk their lives for us.

……

The road ahead will be long. Our climb will be steep. We may not get there in one year or even in one term. But, America, I have never been more hopeful than I am tonight that we will get there.

I promise you, we as a people will get there.

小题1: This passage is most probably taken from a (an)_______.

A.report

B.lecture

C.speech

D.announcement小题2:Which of the following statements is true according to the passage?

A.The speaker has won an election campaign which is quite important for him.

B.The speaker feels they are unlikely to make their ancestors’ dream come true.

C.The speaker thinks America is a nation that lacks democracy.

D.The speaker’s campaign was launched in the nation’s capital.小题3:The purpose of the speaker is to do the following except______.

A.expressing thanks to his supporters

B.discussing the challenges that lie ahead

C.urging Americans to unite for the bright future of the nation

D.warning the people against two terrible wars in store小题4:We can see from the passage that the speaker is a (an)_______person.

A.unreliable

B.determined

C.narrow-minded

D.humorous