问题 计算题

如图所示,绝缘水平面上静止着两个质量均为m、电荷量均为+Q的物体AB(AB均可视为质点),它们间的距离为r,与水平面间的动摩擦因数均为μ,求:                                  

(1)A受的摩擦力为多大?

(2)如果将A的电荷量增至+4Q,两物体开始运动,当它们的加速度第一次为零时,AB各运动了多远距离?

答案

解:(1)由A受力平衡,得:

A受的摩擦力为FAFk

(2)当加速度第一次为零时,库仑力和滑动摩擦力大小相等

μmgkr'=2Q

间距增大了2Qr

AB的质量相等,所以加速度在这段时间内的任何时刻总是等大反向,因此AB运动的距离相等,各运动了Q

阅读理解

阅读理解。

                                                       

     TEN-YEAR-OLD Barack Obama was one of the only three black students at his school in Hawaii, US.

He felt very different from most other students. White girls wanted to touch his hair. A white boy asked

him whether his father ate people.

     "I lied (说谎) to them thin my father was a Kenyan prince (王子). But I kept asking myself who I am,"

said Obama.

     However, 37 years later, the boy made history. Last Wednesday Obama became the first black president

in US history.

     Obama was born in 1961. to an African father and a white American woman from a small town in the

US. He grew up in Indonesia and Hawaii. This unusual background made him wonder who he was. He once

turned to wines to help forget this question.

     With the help of his friends, Obama finally turned his life around at college. His hard work made him a

star at Harvard. Later, he became only the third black senator (参议员) in US history. During his race to the

highest post in the US. Obama talked about his background. He called for a United States of America, rather

than a white America or a black America.

     "Obama's success has made Martin Luther King's dream come true. That is: A man should not be judged

(判断) by the color of his skin (皮肤), but by the content of his character," wrote ABC news.

1. Where can we find this passage?

A. In a newspaper.

B. In a novel.

C. In a history book.

D. In a poster.

2. Why did Obama feel different from most other students at school?

A. Because white girls always touched his hair.

B. Because his father looked terrible.

C. Because there were few black students like him in the school.

D. Because his parents were both Africans.

3. we can infer (推断) from the passage that _____.

A. Martin Luther King fought for his dream and he succeeded.

B. Obama has the same dream as Martin Luther King.

C. Obama was once bothered (困扰) by the question where he was born.

D. Obama became the president at the age of 37.

4. In Obama's opinion, what would "a United States of America" be like?

A. It would be a country made up of fifty states.

B. It would be ruled by the black people.

C. It would be ruled by the white people.

D. It would be a country of all Americans.

5. What is the best title of this passage?

A. A Man who Changed American History.

B. Kenyan Prince.

C. A Top Student of Harvard.

D. Who Am I?

选择题