问题 多选题

如图所示,下列关于机械能是否守恒的判断正确的是(  )

A.

物体A将弹簧压缩的过程中,A机械能守恒

B.

A置于光滑水平面,物体B沿光滑斜面下滑,物体B机械能守恒

C.

不计任何阻力时A加速下落,B加速上升过程中,A、B机械能守恒

D.

小球沿水平面做匀速圆锥摆运动时,小球的机械能守恒

答案

A、在物体A压缩弹簧的过程中,弹簧和物体A组成的系统,只有重力和弹力做功,系统机械能守恒.对于A,由于弹簧的弹性势能在增加,则A的机械能减小.故A错误.

B、物块B沿A下滑的过程中,A要向后退,A、B组成的系统,只有重力做功,机械能守恒,由于A的机械能增大,所以B的机械能不守恒,在减小.故B错误.

C、对A、B组成的系统,不计空气阻力,只有重力做功,A、B组成的系统机械能守恒.故C正确.

D、小球在做圆锥摆的过程中,重力势能和动能都不变,机械能守恒.故D正确.

故选:CD.

选择题
阅读理解

John and Booby joined a wholesale company together just after graduation from college the same year. Both worked very hard. After several years, however, the boss promoted(提升)Bobby to the position of manager but John remained an ordinary employee(雇员). John could not take it any more, so he sent his resignation(辞职信) and complained that the boss did not value(重视)hard working workers but only promoted those who said good words of him.

The boss knew that John worked very hard for the years. He thought for a moment and said, “Thank you for telling me, but I have a request. I hope you will do one more thing for our company before you leave. Perhaps you will change your mind and take back your resignation.”

John agreed. The boss asked him to go and find out anyone selling watermelons in the market. John went and returned soon. He said he had found out a man selling watermelons. The boss asked how much per kg, John shook his head and went back to the market to ask and returned to tell the boss $1.2 per kg.

The boss told John to wait a second, and he called Bobby to go and find anyone selling watermelons in the market. Bobby went, returned and said only one person sold watermelons. $1.2 for a kg, $10 for 10kg and his watermelons added up to 340. On the table 58 melons, every melon weighs about 2 kg, bought from the south two days ago and they were fresh, red and good quality.

John was surprised and realized the difference between himself and Bobby. He decided not to go away but to learn from Bobby.

The story tells us that a more successful person is more concerned and thinks more. Chances are there in the daily details. For the same thing, a person sees one year ahead, while another sees only tomorrow. The difference between a year and a day is 365 times, how could you win?

小题1:John wanted to stop his job in his company because_________.

A.Bobby was promoted but he was not

B.he didn’t want to say good words of his boss

C.he thought the boss didn’t take his hard work seriously

D.he didn’t want to be an ordinary employee小题2:The boss asked John to find out where watermelons were sold in order to _________.

A.persuade him to stay in the company

B.prove he was a hardworking worker

C.buy some watermelons for the company

D.prove his abilities at work for himself小题3:From what Bobby did, we can see that he was promoted only because _________.

A.he was a more concerned businessman

B.he knew how to do business

C.he was more experienced at work

D.he knew details about the watermelon market小题4:We can conclude from the story that _________.

A.one should stop his job if the boss doesn’t think much of his work

B.it’s necessary to know the market when you buy a watermelon

C.detail plays a very important part in one’s success

D.one should get along well with his boss if he wants to be promoted小题5:What can be the best title of the passage?

A.More Concerned, More Successful

B.How to Buy Watermelons

C.Who Is More Successful

D.Proud John