问题 选择题

如图所示,物体P以一定的初速度v沿光滑水平面向右运动,与一个右端固定的轻质弹簧相撞,并被弹簧反向弹回,若弹簧在被压缩过程中始终遵守胡克定律,那么在P与弹簧发生相互作用的整个过程中(  )

A.P的加速度大小不断变化.方向也不断变化

B.P的加速度大小不断变化,但方向只改一次

C.P的加速度大小不断改变,当加速度数值最大时,速度最小

D.有一段过程,P的加速度逐渐增大,速度也逐渐增大

答案

A、木块向右运动压缩弹簧,弹力方向始终向左,根据胡克定律可知:弹簧的弹力与弹簧压缩的长度成正比,当木块向右压缩弹簧时,弹力逐渐增大,加速度逐渐增大;

木块被弹簧弹回向左运动,弹簧恢复原长,加速度逐渐减小,故A错误;

B、根据A中分析知B错误;

C、木块水平方向只受到弹簧的弹力,根据胡克定律可知:弹簧的弹力与弹簧压缩的长度成正比.当木块向右压缩弹簧时,弹力逐渐增大,根据牛顿第二定律知,加速度逐渐增大,做变减速运动,当速度减为零时,速度最小,弹力最大,加速度最大,所以C正确;

D、木块反弹过程向左运动,弹簧恢复原长,弹力减小,加速度逐渐减小,但速度逐渐增大,恰好恢复原长时,速度最大,加速度最小为零,所以D错误;

故选C.

阅读理解

B

The first time I saw Carlos I would never have believed he was going to change my life. I had my arms full of books and I was tearing into the classroom when I ran into something solid. It was Carlos.

“My, you’re tall,” he said.

Of course, the class began to laugh. Angry, I walked to my seat without a word.

I glanced back to see if Reed Harrington was laughing with the rest. That would be the last straw. But Reed was studying chemistry and did not seem to be aware of anything else. I didn’t know why I considered Reed my friend. Maybe just because he was a good two inches taller than I. Anyway, every time I blew out my birthday candles and made a wish, it was for a date with Reed Harrington. “Take that seat,” Mr. McCarthy told the proud newcomer Carlos, pointing to the only empty one, in the back of the room.

Carlos smiled broadly.“But I need a couple of dictionaries.” Again the class laughed, but now they were laughing with Carlos, not at him. He had been here only 10 minutes and already he had them on his side.

It was the school elections that made me think of Carlos again. Reed Harrington was voted president and Carlos vicepresident.“How come?” I kept asking myself.“How come this shrimp(虾) who’s only been in town for a little over a month gets to be so popular.”

So on that morning, I stopped Carlos and said,“It doesn’t seem to bother you—being short.” He looked up at me.“Of course I mind being short. But there isn’t anything I can do about it. When I realized I was going to have to spend my life in this undersized skin, I just decided to make the best of it and concentrate on being myself.”“You seem to get along great,” I admitted.“But what about me? Nobody wants to date a girl taller than he is.”“The trouble with you is you’re afraid to be yourself. You’re smart. And you could be pretty. In fact, you might be more than pretty.” I felt myself turning red...

55.It’s a story about______.

A.a tall boy and a short girl      B.a short boy and a tall girl

C.a tall boy and a tall girl        D.a short boy and a short girl

56.From the story, we can see the boy Carlos is______.

A.handsome and humorous       B.humorous and confident

C.handsome and popular         D.honest and popular

57.The underlined sentence “That would be the last straw” suggests that the writer______.

A.cared much about Reed’s attitude

B.thought Reed was different from the rest

C.couldn’t stand Carlos making fun of her

D.hoped Reed was unaware of the event

58.According to the passage, what happened in the end?

A.Carlos and the author hurt each other.

B.Reed and the author fell into love with each other.

C.The author changed attitude and turned to be confident.

D.Carlos was finally elected president of student committee.

单项选择题