I've been reading "The Universe in a Nutshell" and there is one thing I don't understand very well. What is the relationship between spacetime, acceleration, and gravity? Please explain to a level of my understanding: I'm in 9th grade. More specifically, what I don't get is why the "equivalence between acceleration and gravity" would work if spacetime was curved (or why it wouldn't if spacetime wasn't curved). If my question isn't clear enough, please tell me.