问题 计算题

一根长为L、质量不计的硬杆OA,杆的中点C及A端各固定一个质量均为m的小球,杆、球系统可在竖直平面内绕O端的水平轴转动,如图所示。若开始时杆处于水平位置,并由静止释放,当该系统在转动过程中通过竖直位置时,A端小球的速度vA为多大?中点小球的机械能比在水平位置时减少了多少?

答案

解:在转动过程中,A、C两球的角速度相同,设A球的速度为vA,C球的速度为vC,则有vA=2vC     ①

以A、C和杆组成的系统机械能守恒,由机械能守恒定律,并选最低点为零势能参考平面,则有

E1=mgL+mgL=2mgL

E1=E2,即   ②

①②两式结合可以求出

小球C的机械能减少量为

阅读理解

     To some people, happiness is something to do with simplicity, and it is the ability to extract pleasure

from the simplest things-such as a peach stone (桃核). It is obvious that it has nothing to do with success.

     For Sir Henry Stewart, it was certainly successful. I remember some ten years ago when he was made King's Counsel, Amos and I went to congratulate him. We smiled happily; he merely looked as miserable

as though he'd received a punishment. He took his success as a child does his medicine. And not one of

his achievements brought even a sign of a smile to his tired eyes.

     I asked him one day what it was like to achieve all one's ambitions. He looked down at his roses and

went on watering them. Then he said, "The only value in achieving one's ambitions is that you then realize

that they are not worth achieving."

     Yesterday, I was passing his house and he called me, "Come and see, Jan. Look! I have done it at last! "There he was, holding a small box of earth in his hand. I observed three tiny shoots out of it.

     "And there were only three!" he said, his eyes laughing to heaven.

     "Three what?" I asked.

     "Peach stones," he replied."I've always wanted to make peach stones grow, even since I was a child,

when I used to take them home after a party, or as a man after a banquet. And I used to plant them, and

then forgot where I planted them.But now at last I have done it. I had only three stones, and three shoots," he counted.

     Then Sir Henry ran off, calling for his wife to come and see his achievement-his achievement of

simplicity.

1. From what fact could we see Sir Henry was successful?

A. He felt happy from the simplest things.

B. He was made King's Counsel.

C. He was able to make the peach stones shoot.

D. He didn't take his success seriously.

2. The author compares Sir Henry's attitude towards his success to a child's taking medicine________.

A. to show that Sir Henry felt miserable when he was promoted

B. to prove that Sir Henry didn't want to be successful

C. to suggest that Sir Henry's future job would be hard to do

D. to tell us that Sir Henry took his success as a tiny thing in his life

3. According to Sir Henry Stewart, the value in achieving one's ambitions is________.

A. that he discovers it valueless

B. that he finds it worth his efforts

C. that he has achieved it

D. that he fails to realize it

4. The writer's opinion about Sir Henry's attitude towards success is________.

A. criticism  

B. disapproval  

C. admiration  

D. tease

单项选择题