问题 多选题

在平直的轨道上甲、乙两物体相距为L,乙在前,它们同时同向开始运动.甲以初速度v1,加速度a1做匀加速运动;乙做初速度为零,加速度为a2的匀加速运动,假定甲能从乙旁边通过,而且不影响,下列情况可能发生的是(  )

A.当a1=a2时,甲乙只能相遇一次

B.当a1>a2时,甲乙可能相遇两次

C.当a1>a2时,甲乙只能相遇一次

D.当a1<a2时,甲乙可能相遇两次

答案

A、甲从乙的旁边通过说明相遇时甲的速度大于乙的速度,若a1=a2,则以后甲的速度将都大于乙的速度,故不会再次相遇,故A正确,

B、若a1>a2B则甲经过乙的旁边以后,甲的速度增加更快,故甲将一直在乙的前面,故B错误,C正确;

D、若a1<a2,则此后某一时刻乙的速度一定会大于甲的速度,故乙将会追上甲,甲乙将再次相遇,故能相遇两次,故D正确;

故选ACD.

阅读理解

Once upon a time in a land far away, there was a wonderful old man who loved everything:animals, spiders, insects.

One day while walking through the woods, the nice old man found a cocoon(茧)of a butterfly.He took it home.A few days later, a small opening appeared; he sat and watched the butterfly for several hours as it struggled to force its body through that little hole.Then it seemed to stop making any progress.It appeared as if it had gotten as far as it could and it could go no farther.Then the man decided to help the butterfly, so he took a pair of scissors and cut off the remaining bit of the cocoon.

The butterfly then appeared easily.

But it had a swollen body and small, fragile wings.The man continued to watch the butterfly because he expected that, at any moment, the wings would enlarge and expand to be able to support the body, which would be smaller in time.Neither happened! In fact, the butterfly spent the rest of its life crawling (慢慢爬行)around with a swollen body and fragile wings.

It never was able to fly.

What the man in his kindness and hurry did not understand was that the limited cocoon and the struggle required for the butterfly to get through the tiny opening were Nature's way of forcing fluid from the body of the butterfly into its wings so that it would be ready for flight once it achieved its freedom from the cocoon.

Sometimes struggles are exactly what we need in our life.If we were allowed to go through our life without any obstacles and difficulties, it would disable us.We would not be as strong as what we could have been.And we could never fly.

小题1:The butterfly could never fly because _______________.

A.it was too fat and strong.

B.it loved crawling around.

C.it got out of the cocoon easily.

D.it didn’t get through the cocoon.小题2:The text is most likely to be found in a book about_______________.

A.popular science

B.humorous stories.

C.successful people

D.philosophyevents小题3:What is the implied meaning of the passage?

A.Life without any obstacles and difficulties is perfect.

B.Obstacles and difficulties contribute to success.

C.Never help a butterfly to get out of the cocoon.

D.A butterfly can never fly without going through the cocoon

多项选择题