问题 选择题

质量为m的物体,以g/3 的加速度由静止竖直下落高度h,在此过程中下列说法中正确的是

A.物体的重力势能减少mgh/3

B.物体的机械能减少2mgh/3

C.物体的动能增加mgh/3

D.重力对物体做功mgh/3

答案

BC

题目分析:下落过程中重力做功为

因物体的加速度为,故说明物体受阻力作用,由牛顿第二定律可知,

解得,重力做功; 阻力做功

重力做功等于重力势能的改变量,重力做正功,故重力势能减小,故AD错误;

机械能的变化量等于除重力以外的力做的功,所以机械能减少,B正确;

由动能定理可得动能的改变量,所以物体的动能增加,C正确;

故选BC

点评:由于部分同学没弄明白功和能的关系,导致在解答中出现问题;应注意重力做功等于重力势能的改变量;而合力的功等于动能的改变量;阻力的功消耗机械能.

阅读理解

An important question about eating out is who pays for the meal. If a friend of yours asks you to have lunch with him, you may say something like this, “I’m afraid it’ll have to be someplace cheap, as I have little money.” The other person may say, “OK, I’ll meet you at McDonald’s.”

This means that the two agree to go Dutch, that is, each person pays for himself. He may also say, “Oh, no, I want to take you to lunch at Smith’s,” or “I want you to try the Chinese dumplings there. They are great.” This means the person wants to pay for both of you. If you feel friendly towards this person, you can go with him and you needn’t pay for the meal. You may just say, “Thank you. That would be very nice.”

American custom(风俗) about who pays for dates(约会) are much the same as in other parts of the world. In the old days, American women wanted men to pay for all the meals. But, today, a university girl or a woman in business world will usually pay her way during the date. If a man asks her for a dance outside the working hours, it means “Come, as my guest(客人).” So as you can see, it is a polite thing to make the question clear at the very beginning.

小题1:The passage mainly tells us ________.

A.how to eat out

B.where to eat out

C.what to eat out

D.who pays for the meal in different situation小题2:The underlined phrase “go Dutch” in this passage means “________” in Chinese.

A.去订餐

B.共同就餐

C.去饭店

D.各自付款小题3:Sometimes your friend takes you to lunch. It means _________.

A.he’s going to pay for your meal

B.he’s going to lend some money to you

C.he can’t understand you

D.he’ll be angry with you小题4:In America, some girls and women _________ now.

A.ask men to pay for their clothes

B.try to pay for their own meals

C.never have anything outside

D.try to pay for the men’s meals小题5:We can infer(推断) if a man asks a woman for a dance outside the working hours _________.

A.the man will pay for it

B.the woman will pay for it

C.they will go Dutch

D.another person will pay for them

问答题 简答题