如图所示，在大小为250 N的拉力F作用下，滑轮组将400N的重物提_爱下问搜题

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问题选择题

如图所示，在大小为250 N的拉力F作用下，滑轮组将400N的重物提升了1m．在此过程中

A．做的总功是400 J

B．做的有用功是250 J

C．滑轮组的机械效率是80％

D．滑轮组的机械效率是62.5％

答案

答案：C

在大小为250 N的拉力F作用下，滑轮组将400N的重物提升了1m．在此过程中

做的总功是W="FS=250" N *2m="500" J; 做的有用功是W有="Gh=400N" *1m="400" J滑轮组的机械效率是400 J/500 J=80％.所以选C。

单项选择题

能将高级语言编写的源程序转换为目标程序的软件是

A．汇编程序

B．编辑程序

C．解释程序

D．编译程序

完形填空

Robeson was born in a very poor family. At seven, he had to pick coal in a deserted mine near his home, and then he sold_ _he had picked and earned a few coins to help his parents. He had_  schooling, for being so poor, how could they_ _school fees?
When he was fifteen, he worked     a servant in a school.Looking at other children studying in the classroom，he felt_  _for himself.How he__ _to have the same chance! He decided to study by__ _.In the daytime, after the sweeping and cleaning was over, he__ _stand by the window outside the classroom trying to catch what the teacher said. At night, he tried his best to remember what he_ _during the day. He worked_ _hard at his lessons that he sometimes had just three or four hours to sleep. The more he learned, the greater_ _he showed in his lessons. A math teacher discovered him and came to like this diligent boy and_ __him to sit at the back of the class. In one exam, he was the_ _one in the whole school who reached the highest grade. He would have been given the scholarship if he__ _a regular student of the school.
Robeson_ _through six long years with his study of math and wrote several articles which captured the_   of some university professors.They admired his talent__ __his diligence.To give him a good chance, they hired him as a librarian and_ _him free guidance.Robeson felt_ _, for he was sure that before him there was a broad road_ _success.

A．the teacher

C．his parents

D．his schoolmates

A．had learned

B．has been taught

D．had been written down

B．according to

D．in spite of

D．disappointed

B．coming from

D．covered with