看图片，写单词。

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    New Year's Day is coming. The students in Class 8 are preparing for a New Year's party. Cindy and Mary are malting red paper birds They want to put them outside (在……外面) the classroom  Lily and Alice are cleaning the desks and chairs They want a clean classroom. There are some pictures on the walls and there are also some beautiful flowers on the desks. Some boy students are learning taijiquan in the playground. They want to show (展示) it at the party. Where are Jack and Nancy? Oh, they are still in the shop. They are buying some food and drinks for the party. "We are very happy because we can have a big party," says Mike, the monitor (班长).

如图所示，重20N的物体放在粗糙水平面上，用F＝8N的力斜向下推物体．F与水平面成30°角，物体与水平面间的动摩擦因数μ＝0.5，则(　　)