问题 计算题

(8分)长为L的绳一端系于O点,另一端系一质量为3m的小球,如图所示,质量为m的子弹水平射入小球并留在其内,小球恰好能过最高处(O点上方L处),求子弹的初速度.

答案

题目分析:小球恰好能通过最高点B,知在最高点轨道对球的弹力为零,重力提供向心力,根据牛顿第二定律可得:,解得

小球从A点到B点,只有重力做功,机械能守恒,则由机械能守恒定律可得:

,解得

子弹水平射入小球并留在其内瞬间,小球与子弹组成的系统动量守恒,则

解得

故子弹的初速度

阅读理解

Have you ever heard of Back-to-Front Day? It is a festival in Britain. On that day, the adults have to go to school. And the children? They are the policemen and firemen; they sit in the offices and call New York and Paris; they are doctors and nurses.

“Stand in the corner, Mr. Bass!” said Mary. Mary was six and had a class of twenty-three men and fifteen women. They were making too much noise.

John was a policeman. Though his uniform was too big, he walked down the street with pride. “Hm, why are you not at school?” “Oh, I ... I have something important to do,” said the man.   “Fiddlesticks (胡说)!” said John angrily. He took out his notebook. “Name?” he asked. “Mr. Green.” John wrote it down and blew his whistle (吹口哨) and a police car appeared. “I have a truant here,” said John. “Drive him to school!”

At the hospital a small doctor climbed on one chair in order to examine (检查) Mrs. Brown’s throat, and Nurse Jenny took Granny Lisa’s temperature.

The best part of Back-to-Front Day is in the evening. The children put the adults to bed, very early. They tell them a story, too — after the adults wash their faces and brush their teeth.

How interesting Back-to-Front Day is!

小题1:The adults have to ____ on Back-to-Front Day.

A.look after their children well

B.work in many different fields

C.go to school

D.work harder小题2:How many students were there in Mary’s class?

A.15.

B.23.

C.38.

D.44.小题3:The underlined word “truant” means “______” in Chinese.

A.逃学者

B.落伍者

C.开拓者

D.爱国者小题4:Where were Mrs. Brown and Granny Lisa?

A.In the street.

B.At the hospital.

C.At school.

D.In the office.小题5:Which of the following is TRUE according to the fifth paragraph?

A.The adults sleep late on that day.

B.The children tell stories to the adults.

C.The adults help the children brush their teeth.

D.The children help the adults wash their faces.

单项选择题