（1）如图所示，一皮球不慎掉进水中漂浮在水面上，试作出皮球所受力的示意图。（2）如图所示，请在A点画出吊起重物G所用最小的力和这个力的力臂。
分析：先由物体静止，判断足球受平衡力作用；再对足球进行受力分析，找出足球受到的力，然后根据平衡力的条件，判断出所受力的大小方向关系，画出力的示意图．根据杠杆平衡条件，当杠杆平衡时，动力臂越长，动力越小．
解答：解：足球静止，所以重力和浮力是一对平衡力，大小相等，方向相反，作用在同一直线上．故足球所受力的示意图如图所示：
分析：
由图可知，吊起重物G，动力作用在A点，方向与OA垂直，力臂最长，此时动力最小．所以所用最小的力和这个力的力臂如图所示：
| 阅读理解。 | ||||||||||
| People love oranges. They are sweet and juicy. They are in sections so it's easy to eat them. Some oranges don't have any seeds. Some have a thick skin and some have a thin skin. The orange tree is very beautiful. It has a lot of shiny green leaves. The small white flowers smell very sweet. An orange tree has flowers and fruits at the same time. There were orange trees forty hundred years ago. The oranges were very small, not like the ones today. The earliest orange trees probably came from China. Many different kinds of oranges are raised there today. Farmers in other parts of Asia and the Middle East learned to raise oranges from the Chinese. Then they taught Europeans. The Spanish planted orange trees in the New World (North and South America) and took them to Florida first. "Orange" is both a fruit and a color. The color of oranges is so beautiful that in English we use the name of the fruit for its color. | ||||||||||
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