问题 填空题

如图所示,工人利用动滑轮将沙匀速向上提升2m,已知沙重180N,动滑轮和桶总重重20N,绳重和摩擦均不计。则在此过程中工人做的有用功为     J,该装置的机械效率为   %,就如何提高该装置的效率提出一点建议                   

答案

360      90     增加物体的重

题目分析:(1)动滑轮可看做是绳子股数为2的滑轮组,绳重和摩擦均不计时,

工人做的有用功:

绳子末端的拉力,工人做总功:

该装置的机械效率

(2)在提升的重物相同时,可以减小动滑轮的重力、减轻桶重或加润滑油,提高机械效率;

在机械装置不变时,可以增加提起的沙子的重,提高机械效率.

点评:本题考查了功、机械效率的计算和提高机械效率的方法,关键是能体会清楚有用功、额外功、总功的含义。

阅读理解

If you ask most Americans, they would say their favorite season of the year is summer. The weather is warm. They do not have to wear heavy clothes to keep warm. Young people do not have to go to school. They can do many activities outside, like playing sports and swimming at the beach or the pool. They like the sunshine during the day and the warm summer nights. People have written and recorded hundreds of songs about summer. These are some of our favorites.

One of the most famous songs about summer is from George Gershwin’s opera “Porgy and Bess”. He wrote the music in nineteen thirty-five. The opera takes place in the southern United States. It opens with these words: “Summertime and the living’ is easy. Fish are jumping’ and the cotton is high.” Leontyne Price sings the song.

The nineteen fifties and sixties produced many songs about teenagers enjoying their summer vacation from school. The songs are about having fun, swimming in the ocean, driving in cars. However, for some teenagers, summer vacation was not all fun and games. Some of them had to work to earn money.

小题1:Why did people write and record so many songs about summer?

A.Summer is warm.

B.They do not have to wear heavy clothes to keep warm.

C.They can do many activities outside.

D.They like summer.小题2:Which of following statements is NOT true?

A.There are 1950s or 1960s songs about teenagers enjoying their summer vacation

from school.

B.Many songs are about teenagers having fun in summer.

C.Teenagers have not only fun and games but other things in summer.

D.Some teenagers have to work to earn money in summer小题3:Please choose an appropriate title for the passage.

A.Summer.

B.Children in Summer.

C.Summer and Children.

D.Songs about Summer.

单项选择题