Jcoasters, please stop speaking. You're 100% wrong on almost everything you've said.
Yes Earth is curved, and no that doesn't mean everything on Earth is too. Nothing will be "perfectly" straight...if you look at razor blades under a microscope, you will see that the edge is quite rough. So these sediment flats and things like that won't be perfect flat. However, you have to think within reason, and that's what you're not doing at all right now. Lets please get past that part of your argument, because it's incredibly silly and illogical.
Just because Earth is curved doesn't mean everything on Earth is. Consider a flat line ontop of a circle. If that were earth and the line was long enough, you'd notice that the ends are higher than the middle, assuming a perfectly round Earth (which is not the case). Generally, things like steel are "perfectly" straight, and the only reason I put perfectly in quotations is because of what I said in the last paragraph. I know of at least one case where engineers had to design something so long that they had to follow the curve of the Earth. There's some Navy engineering facility in...Virginia or something where they have a tube that's like 1/4 mile long, and they had to make it curve say an inch over the length. Those numbers are from memory, and it's been a while, but the point is they had to intentionally make it curved.
And are you really arguing anything other than the standard definition of airtime on a roller coaster site? Why would you use that term if you mean anything aside from forces less than say ~.2G on the vertical axis? Also, what you said about Kingda Ka is wrong. As someone else noted, if you stopped while going down, you'd experience 1G pushing you forward into the harness...the normal sensation of gravity. As is, you're at ~0G for the vertical section. 0G artime can be created either through hill curvature with a given radius, or a slope of 90*.
And gravity is not curved, at least not in the way you're saying. Earth is a circle, and gravity points straight toward the center. It's really not a hard concept. What you said about gravity being curved because Earth is curved makes no sense at all.
Jcoasters, I can prove gravity exists (not that I should even reply to that ludicrous reasoning). Jump up. I guarantee you will come back down. Proof complete.
Hey look, a bunch of straight lines!