同学们可在学完第九章《运动和力》后再回头看此篇，理解“力”这一概念会更深一层．苹果在树上随风荡了阵秋千．满脸通红、气喘嘘嘘地说：“哎，想当初．我常在枝头随风飘摇，而如今只微晃几下，就累得够呛．这是怎么回事？”地球听见了，对他说：“你大胖了，用牛顿大师的话? The students can look back at this chapter after completing the ninth chapter “Sports and Power” and understand that the concept of “force” will go deeper. The apple sways on the tree with a swing. With a flushed face and breathlessness, I said, “Oh, I want to be in the beginning. I’m always on the branch with the wind, but now I only sway a few times and I’m so tired. How is this going?” Earth heard, right. He said: "You are fat, use Newton’s words?