At the moment, I am reading the book Relativity, written by Albert Einstein regarding his special and general theory of relativity. In his introduction to general relativity, he poses a thought experiment: Suppose we have a large chest in space far away from any appreciable mass (stars, planets, etc.). Inside of this chest, we have an man (it's a large chest) standing on the floor of the chest with strings holding him to the chest floor. Furthermore, he is holding a rock in his hand. Now let's suppose there is a hook on the outside of the lid of the chest that allows some immaterial being to pull the chest "upward" relative to its current plane with a constant force F. The book describes how if he let go of the rock, it would fall to the floor. Why would it not fall upward? Here is the direct quote from the book: Can someone please explain this bolded excerpt? Why is this true?