问题 选择题

如图所示,放在光滑斜面上的小球,一端系于固定的O点,现用外力缓慢将斜面在水平桌面上向左推移,使小球上升(最高点足够高),在斜面运动过程中,球对绳的拉力将(   )

A.先增大后减小

B.先减小后增大

C.一直增大

D.一直减小

答案

答案:B

题目分析:如图所示为球的重力作用效果的分解图,重力不变,斜面的支持力方向不变,因此,所构成的矢量的平行四边形就随着小球的上升而不断地发生变化。从图中看出,随球的上升,绳子的拉力从,大小是先减小后增大。

故选B

点评:中等难度。动态平衡是指平衡问题中的一部分力是变力,是动态力,力的大小和方向均要发生变化,所以叫动态平衡,这是力平衡问题中的一类难题.常用的方法是解析式法和画图法,本题用画图法分析。

阅读理解

阅读理解。

     When you go to see a doctor, he will write you a note (处方) to take to the chemist for some

medicine. Chemists are usually good at reading doctors' notes.But sometimes doctors write too badly

and even the chemist can not read them.

      One day a man wrote to a doctor to invite (邀请) him to have dinner at his house. The doctor wrote

a reply (回复), but he wrote too badly and the man could not read it.

     "What shall I do?" he asked his wife, "I don't know whether he is going to come or not. I don't want

to call him and say that I don't understand him."

     His wife thought for a while and then she had an idea. "Take it to the chemist," she said, "He will be

able to(能) read it for us."

     "Thank you," said her husband. "That's a good idea." He went to the chemist's shop and gave the

doctor's "note" to him. The chemist looked at it for a long time.

      "Could you wait a moment, sir?" he said. Then he went to the back of his shop. After a few minutes

he came back, carrying a large bottle. He gave the bottle to the man.

     "Take one spoonful (一匙) every day."he said.

1. What did the doctor do when he was invited to have dinner? [ ]

A. He said he would come on time.

B. He wrote the man a reply.

C. He was very glad.

D. He wrote a note to the chemist.

2. Why couldn't the man understand the "note"?[ ]

A. Because the doctor wrote badly.

B. Because the man didn't know much about medicine.

C. Because the doctor didn't like to go.

D. Because the man never went to school

3. The wife wanted her husband to _____.[ ]

A. call the doctor to have dinner with them

B. go to the chemist and get some medicine

C. take the reply to the chemist for help

D. read it again

4. In this story, the word "chemist" means "______" in Chinese.  [ ]

A. 护士

B. 化验师

C. 药剂师

D. 药房

5. From this story, we can infer(推测) ______. [ ]

A. The man took the bottle of medicine back home.

B. The chemist could read the doctor's note.

C. The chemist couldn't read the doctor's note, either.

D. The chemist helped the man at last.

单项选择题