1) Air is a gas. We all know that gases expand until they reach some sort of barrier. In an RE, this barrier is the Ozone layer/gravitational pull. In a Flat Earth...
No barrier. Thusly, air would expand and flow off of the edge of the Flat Earth as it is propelled through the universe. We'd be leaking air... but we arn't!
Explain why we arn't all suffocating.
(If you say the same force that propels us through the universe also keeps it in, you'd be wrong because that force would push the air easier than the Earth, and would take away air from us)
2) If we are accelerating, air pressure would increase rapidly as the Earth would constantly apply more pressure to the air, and would slowly squeeze it - if the air somehow had a way of being held in on the sides. Aircraft's cielings at which they could fly would decrease, and we would slowly be crushed under air pressure. Eventually, the air would be compressed to the point where it would combust, and we'd all die.
Explain why this does not happen, if the Earth is flat.