问题 选择题

两木块A、B用一轻弹簧拴接,静置于水平地面上,如图中甲所示,现用一竖直向上的恒力F拉动木板A,使木块A由静止向上做直线运动,如图中乙所示,当木块A运动到最高点时,木块B恰好要离开地面,在这一过程中,下列说法中正确的是(设此过程弹簧始终处于弹性限度内)

A.木块A的加速度先增大后减小

B.弹簧的弹性势能先增大后减小

C.木块A的动能先增大后减小

D.两木块A、B和轻弹簧组成的系统机械能先增大后减小

答案

答案:C

题目分析:木块A在拉力F作用下由静止最后到达最高点时也静止,说明木块A向上先加速后减速,选项A错误、C正确;木块A向上运动的过程中,弹簧压缩量先逐渐减小,后被逐渐拉伸,其弹性势能先减小后增大,选项B错误;由于拉力F始终对两木块A、B和轻弹簧组成的系统做正功,故系统机械能增加,选项D错误,故选C

点评:难度较小,加速度的变化根据合外力变化判断,弹性势能变化根据弹力做功判断,机械能变化根据除了重力和弹力以外其它力做功判断

阅读理解

阅读理解

     "Hey, Jenna, do you think we'll still be friends when we're eightytwo?" I asked my friend.

While I waited for Jenna to answer, I started wondering what life would be like without her.

     Suddenly, my thoughts were interrupted. "Of course, we'll still be friends when we're

eightytwo," Jenna announced loudly.

     The next year, in fourth grade, we met Jamie. Since she lived in the same neighborhood as

Jenna and me, the three of us soon clicked into a really tight group of friends. Things couldn't

have been better, and I thought even time couldn't pull us apart, but that is where I was sadly

mistaken.

     The three of us started fighting a lot-and not just small fights where your friend won't return

a CD you let her borrow. Before Christmas, we had a really big fight.  They both said I was

bossy and couldn't keep my mouth closed. I felt helpless and alone.

     I was surprised when Jenna came to my house and gave me a nice Christmas card she had

made for me. I was so sure that she was still disappointed with me, but now I was getting a

really nice card that she even made herself.

     "Wow," I said, breaking the silence as we stood on either side of my front door. "Thanks."

     "Okay... well... I have to go," she said softly.

     The card started off with  "Merry Christmas", but then, farther down the page, it said,  "I am

so glad we're friends. I am sorry about what I said when we were fighting. A fight won't stop us

from being friends. Besides, we said we were going to be friends even when we're eightytwo."

1. Before fourth grade, the author and Jenna________.

A. were strangers

B. were close friends

C. often fought with each other

D. didn't get along well with each other

2. Jenna and Jamie thought the author________.

A. looked after them very well

B. often cheated them

C. didn't help them

D. often ordered them about

3. Why did the author feel surprised when she received a nice Christmas card from Jenna?

A. Because Jenna had never sent her such a nice card before.

B. Because she never thought Jenna still regarded her as a close friend.

C. Because she thought Jenna would come to fight with her again.

D. Because Jenna didn't return her the CD she had borrowed.

4. After the Christmas, the author and Jenna would ________.

A. lose touch with each other

B. go on fighting with each other

C. not speak to each other

D. be close friends again

名词解释