A train stops at a station(车站). A young man wants to come out, but it is raining. A boy is standing under a big umbrella. The young man says to the boy，"Can you go and get us two hamburgers, one for you and one for me? Here are two dollars. " "Great！"says the boy and he goes to buy hamburgers. After some time, the boy is back. He is eating a hamburger. "Where is my hamburger？"asks the young man. "Oh, there is only one hamburger left(剩下). So I'm eating mine. Here is your dollar. "

 ____  ____   ____= ____

以下是小明同学对一些工具及其原理的总结，其中说法正确的是（　　）

如图所示，在水平力F的作用下，重为G的物体保持沿竖直墙壁匀速下滑，物体与墙之间的动摩擦因数为μ，物体所受摩擦力大小为(     )