问题 选择题

如图所示,轻质弹簧上端悬挂于天花板,下端系有质量为M的圆板,处于平衡状态.开始一质量为m的圆环套在弹簧外,与圆板距离为h,让环自由下落撞击圆板,碰撞时间极短,碰后圆环与圆板共同向下运动.使弹簧伸长.那么(  )

A.碰撞过程中环与板系统的机械能守恒

B.碰撞过程中环与板的总动能减小转化为弹簧的弹性势能

C.碰撞后新平衡位置与下落高度h无关

D.碰撞后环与板共同下降的过程中,它们动能的减少量等于弹簧弹性势能的增加量

答案

A、圆环与圆板碰撞过程,时间极短,内力远大于外力,系统总动量守恒,由于碰后速度相同,为完全非弹性碰撞,机械能不守恒,故A错误;

B、圆环与圆板碰撞过程,时间极短,内力远大于外力,系统总动量守恒,由于碰后速度相同,为完全非弹性碰撞,机械能不守恒,减小的机械能转化为内能,故B错误;

C、碰撞后平衡时,有

kx=(m+M)g

即碰撞后新平衡位置与下落高度h无关,故C正确;

D、碰撞后环与板共同下降的过程中,碰撞后环与板共同下降的过程中,它们动能和重力势能的减少量之和等于弹簧弹性势能的增加量,故D错误;

故选C.

多项选择题
阅读理解

I got your letter and was glad to find that you had not forgotten Jourdon, and that you wanted me to come back and live with you again in Tennessee, promising to do better for me than anybody else can.

Although you shot at me twice before I left you, I did not want to hear of your being hurt. It would do me good to go back to the dear old home again and see Allen, Esther, Green, and Lee. Give my love to them all, and tell them I hope we will meet in the better world, if not in this.

I want to know particularly what the good chance is you propose to give me. I am doing tolerably well here in Ohio. I get twenty-five dollars a month, with food and clothing; have a comfortable home for Mandy, the folks call her Mrs. Anderson; and the children—Milly, Jane, and Grundy—go to school and are learning well. The teacher says Grundy has a head for a preacher. We are kindly treated.

Now, if you will write and say what wages you will give me, I will be better able to decide whether it would be to my advantage to move back again. We have concluded to test your sincerity by asking you to send us our wages for the time we served you. This will make us forget and forgive old scores and rely on your justice and friendship in the future. I served you faithfully for thirty-two years, and Mandy twenty years. At twenty-five dollars a month for me, and two dollars a week for Mandy, our earnings would amount to eleven thousand six hundred and eighty dollars. Add to this the interest for the time our wages have been kept back, and deduct(扣除)what you paid for our clothing, and three doctor’s visits to me, and pulling a tooth for Mandy. If you fail to pay us for faithful labors in the past, we can have little faith in your promises in the future.

In answering this letter, please state if there would be any safety for my Milly and Jane, who are now grown up, and both good-looking girls. I would rather stay here and starve—and die, if it should come to that—than have my girls brought to shame by the violence and wickedness of their young Masters. You will also please state if there has been any schools opened for the colored children in your neighborhood. The great desire of my life now is to give my children an education and have them form virtuous habits.

Say howdy to George Carter, and thank him for taking the pistol from you when you were shooting at me.

小题1:According to the passage, the letter was written by Jourdon to his former _______.

A.friend

B.master

C.neighbor

D.relative小题2:Which of the following is RIGHT according to the passage?

A.The family name of this letter writer is Anderson.

B.The writer is paid the same as he was in Tennessee.

C.The writer will certainly get at least 11,680 dollars.

D.Safety rather than education weighs a lot to the writer.小题3:The writer’s description of his present situation implies that _______.

A.he shows his intention of going back in Tennessee

B.he is somewhat richer and does not need to go back

C.his life is relatively good but still needs improvement

D.he is not a little satisfied with his present life in Ohio小题4:The purpose of the writer’s asking for his pay back is _______.

A.to show he needs that amount of money urgently

B.to show he is determined to get what he deserved

C.to test whether the letter receiver is worthy of trust

D.to tell the letter receiver he still has faith in him小题5:From the passage, we can see the writer is very _______.

A.wise

B.stupid

C.greedy

D.generous