Hello fellows round earthers and flat earthers ! To those who don't know me, I'm a french engineering student, so I apologize in advance for any spelling or grammar mistake. I'm not a native english speaker.
Today, we'll talk about relativistic physics and Universal Acceleration, the mechanism used to explain gravity in flat earth theory. So according to the wiki and FAQ, the earth is constantly accelerated at a rate of 9.8 m.s
-2, and this is what explain gravity. It does not violate the laws of physics, as in Einstein's special relativity it is stated that one can accelerate indefinitely without ever reaching the speed of light. In the earth reference frame, we would still experience a constant acceleration, but an outside observer would see the speed of the earth slowly approaching the speed of light.
That is great, but here's the trick : FE'ers just forgot about a crucial part of special relativity : the mass/energy relationship. To put it simply, in special relativity, if you cannot exceed light speed, there's no limit to the kinetic energy of a moving body, it just tends toward infinity as the body approaches the speed of light (we'll call it c for the rest of the discussion). And if the energy of the body rises and tends toward infinity, so does the inertial mass of the body, thanks to the famous equation E=mc
2 ! That's no good for FE, because we see everyday that our masse doesn't increase as times goes. But hey, let's calculate how it would be to live on a accelerating, relativistic flat earth !
We will consider the following system (sorry about my paint skills):

The system Bob+flat earth has a rest mass m0 and is submitted to an acceleration g by the UA. In the reference frame of the flat earth, g is constant. We will calculate the speed of the system in the universe reference frame and we will deduce the increase in mass of the system function of the time.
This webpage was quite handy : (sorry, it's in french, and there is no similar page in the english version):
http://fr.wikipedia.org/wiki/Calculs_relativistes#Le_mouvement_uniform.C3.A9ment_acc.C3.A9l.C3.A9r.C3.A9It give us the formula for calculating the speed of a uniformingly accelerating relativistic body :

Here, V is the speed of the flat earth in the universe reference frame. Now, let's use the relativistic formula of energy to find the variation of mass of our system !

gamma is the Lorenz coefficient, m is the total mass of the system and m_0 is the rest mass. By expressing the Lorenz coefficient and simplifying our formula, we find the following expression :

Now we're getting somewhere ! let's inject the expression of V in this formula :

We simplify it, and there we are : the expression of the mass of the system function of time !

So what does this function looks like ? Well, it is not good for FE, as it clearly tends toward infinity, and bob would feel the effects of the increase in mass on the long term. Lets make a chronology !
t=0s : the flat earth is not moving, bob float in the air.
from t=0 to t=6 months : the earth accelerate upward at 9.8 m.s
-2 ! Bob is happy ! The relativistic effects are still to tiny to notice.
from t=6 months to t=13 months : Bob noticed that something was wrong, as he gained 50% mass in 7 months without getting fat.
from t=13 months to t=3 years : during this period, Bob would not be able to walk, as at the end of the 3 years, his mass will have tripled. Breathing should be difficult. On the flat earth, every trees and buildings would have collapsed. I didn't calculated the effects on the atmospheric pressure, but it would definitely affect it.
from t= 3 years to t=10 years : everything is 9 times heavier. Bob dies, as his heart is not able anymore to pump blood to his brain.
After this, if the earth continues accelerating, the athmosphere will likely be compressed enough to start nuclear fusion, and the earth will become the first pancake-star ever.
Sorry FE'rs. The UA is a myth.