某人从一楼匀速率上到三楼的过程中，下述说法正确的是（ ）
A．地板对人的支持力做功等于人的重力势能的增加
B．地板对人的支持力做功等于人的机械能的增加
C．地板对人做的功为零
D．人克服重力做的功等于其重力势能的增加
人受到的支持力的方向是竖直向上的，但是支持力没有发生位移，所以支持力不对人做功，所以AB错误，C正确；
在上升的过程中，人克服了重力做功，克服重力做的功增加了人的重力势能，所以D正确．
故选CD．
先阅读短文，然后根据短文内容从方框内所给的单词中选择恰当的词（注意词形变化）填入短文空白处，使短文完整、通顺。一词只用一次，一空限填一词。方框中有两个词是多余的。
A student was going to leave the capital to become an official in a city far away. Before he started, he came to say 74 to his teacher. “It is not an 75 job to be a good official,” his teacher warned. “You must be strict 76 yourself and never be careless.” “Don’s worry about me, sir.” the student answered. “I have already 77 one hundred top hats, which will 78 those people quite happy.” “But we are real gentlemen! 79 could a real gentleman do such a thing?” his teacher was a bit angry. “Never forget 80 I taught you in class!” “You are always right, sir. I also hate such things. But sir, 81 no one real gentleman like yourself can be seen in the world now,” said the student. It seemed that he had to do so. After hearing this, the teacher was 82 . “What you said is true!” “Of my one hundred top hats, I have ninety-nine 83 .” the student said to his friend later. |
