问题 单项选择题

下列关于质点的说法中,正确的是()

A.质点是一个理想化模型,实际上并不存在

B.只有体积很小的物体才能看作质点

C.凡轻小的物体,皆可看作质点

D.研究一列火车的过桥时间,可以把这列火车看做质点

答案

参考答案:A

解析:质点是用来代替物体的具有质量的点叫做质点.A、质点是实际物体的一种高度抽象,实际中并不存在.是一种“理想化模型”;正确BC、并不是只有很小的物体才能被看做质点,也不是很大的物体就一定不能被看做质点,关键是看物体的大小和形状在研究物体的运动时是否为次要因素; B、C错误D、研究火车过桥的时间,火车长度和桥长比不能忽略;错误故选A

阅读理解

阅读理解。

      One morning I was woken by my mother's angry voice. I opened my eyes and found it was seven forty,

so I jumped out of bed and rushed to a nearby bus stop without breakfast. Fortunately the bus started going

as soon as I got on it. I was sure I could reach the school before the bell rang.

      However, the bus suddenly stopped. There was something wrong with it and we had to get off and wait

for another one. I felt I couldn't wait and the school was only fifteen minutes' walk, so I ran to the school as

fast as possible. Just at this moment, it began to rain hard.

      I reached the classroom with all my clothes wet through. Mr. Smith, our physics teacher, had begun his

class. I was always afraid of him. I hesitated (犹豫) for a moment, and then I opened the door.

      "It's you again, Jim," Mr. Smith said angrily. "I've told you not to be late, but you …". Then he suddenly

stopped when he noticed my wet clothes. He quickly took off his own jacket and asked me to put it on. "I

don't want you to catch a cold," he said kindly. I went to my seat with Mr. Smith's jacket on. It was a bit

large for me but I felt very warm in it. I found, for the first time, Mr. Smith's physics class so interesting and

I changed my mind, there and then, about being afraid of him.

1. According to this passage, classes in Jim's school probably began at _____.[ ]

A. 9 am

B. 8 am

C. 7 am

D. 7:30 am

2. Why did Jim hesitate before he opened the door? [ ]

A. He was afraid of facing Mr. Smith.

B. He didn't like to have the physics class.

C. Mr. Smith didn't let him go into the classroom.

D. He didn't want to interrupt others.

3. Jim found Mr. Smith physics lesson interesting that day because _____. [ ]

A. he knew physics was very important

B. he wanted to go to famous university

C. he didn't want to fall behind the others

D. he was moved by his physics teacher

4. We can infer from the passage that _____. [ ]

A. Jim usually went to school by bike

B. it was the first time Jim had been late

C. Jim was often late for school

D. Jim's mother also got up late that morning

单项选择题