问题 多选题

小球固定在轻杆的一端,随杆一起绕另一端在竖直平面内作圆周运动,在运动过程中,杆对球的作用力(  )

A.一定是拉力

B.一定是压力

C.可能有时为拉力,有时为压力

D.在某一时刻可能为零

答案

在最低点,重力和杆子拉力的合力提供圆周运动的向心力,杆子表现为拉力.

    设杆子的长度为L,在最高点,若速度v=

gL
,则杆子作用力为0.若速度v>
gL
,杆子表现为拉力.速度v<
gL
,杆子表现为支持力.故C、D正确,A、B错误.

故选CD.

阅读理解

阅读理解

     I was off to go back to work one evening when I saw my two children were busy sewing things on the sewing machine. My 11yearold daughter was, in the midst of her project, going to assist her elder brother

in making a little cushion. I left, and after a few hours returned to find a mess in the kitchen and in the front room, and both children sitting in front of the television.

     Having had a long day, I was too tired to greet them and then I noticed the material my daughter had

used. It had been purchased to make a colorcoordinated (颜色协调的) baby blanket, and now had

chunks (块) cut out of almost every piece of fabric. Not stopping to listen, I exploded at the children and

explained how angry I was at what had been done.

     My daughter listened to me sheepishly, not trying to defend herself at all, but the pain could be seen

written across her face. She retreated to her room, and spent some time there alone before she came out

to say good night and once again apologized for the mistake she had made.

     A few hours later, as I was preparing to go to bed, I found that there on my bed lay a beautiful, little

cushion made out of the forbidden fabric, with the words "I LOVE MOM". Alongside it was a note

apologizing again, and the innocence in which she had taken the fabric.

      To this day, I still get tears in my eyes when I think of how I reacted and still feel the pain of my

actions. It was I who then went to her and apologized for my actions. I display with great pride the

cushion on my bed, and constantly use it to remind me that nothing in this world is greater than a child's

love.

1. The underlined word can be replaced by "________".

A. hopefully came up  

B. hopefully went back

C. quietly came up  

D. quietly went back

2. What did the children do?

A. They cut out the material and made it into a baby blanket.

B. They cut the material into pieces and used them to clean the TV set.

C. They cut the material into pieces on which they wrote apologizing words.

D. They made a cushion with the material to show their love for Mom.

3. What can be inferred from the text?

A. The two children are good at sewing.

B. The two children are introverted (内向的).

C. The mother is often rude to her children.

D. The mother is regretful for what she did.

4. The passage is mainly to tell about________.

A. a mom's tears  

B. a mom's love

C. a child's love  

D. a child's complaint

问答题 简答题