下列现象中，由动能转化为重力势能的是( )

2017年，乌鲁木齐市创建全国文明城市将全面迎检。创建全国文明城市，不是为了争块“牌子”、拿个“荣誉”，而是提升一个城市核心竞争力和综合软实力的过程。这启示我们（    ）

    Hello, I'm Jack. I live in a house. My address is 246 Second Street. It's far from the school. So I go to school by bus every day. But my house is near the park. So I often go to the park on foot. Tom is my friend. He lives in a house, too. It is near the school. So he goes to school on foot every day.

经过初中阶段的学习，同学们已经知道，导体的电阻是导体本身的一种性质，它的大小决定于导体的材料、长度和横截面积(不考虑温度对电阻的影响)。请你用学过的知识和实验方法研究导线的电阻R跟导线的长度L间的定量关系。