In the Flat Earth model, 'gravity', rather than being a force, is the upward acceleration of the Earth. The Earth always accelerates upward at 1g, which is equivalent to the gravitational acceleration in the Round Earth model. Like the force of gravity, the Earth's acceleration causes several commonly observed phenomena in our daily lives.
'Universal Acceleration' is a theory of gravity within the Flat Earth Model. The traditional theory of gravity (e.g. Newton's Law of Universal Gravitation, General Theory of Relativity, etc) is incompatible with the Flat Earth Model because it requires a large, spherical mass pulling objects uniformly toward its center.
Rather than a downward pull due to the presence of mass, the theory of Universal Acceleration asserts that the roughly disk-shaped Earth is accelerating 'upward' at a constant rate of 1g (9.8m/sec^2). This produces the effect commonly referred to as "gravity".
There are two Universal Acceleration models. The first model deals with the Universal Accelerator, which sits underneath the Earth and accelerates anything it touches. The second model deals with Dark Energy, which accelerates all celestial bodies, including the Earth, in the universe. Modern astrophysics accounts that the expansion of the universe is due to Dark Energy.
Velocity of the Earth
According to the Special theory of Relativity, the Earth can accelerate forever without reaching or passing the speed of light. Relative to an observer on Earth, the Earth's acceleration will always be 1g. Relative to an inertial observer in the universe, however, the Earth's acceleration decreases as the its velocity approaches c. It all depends on our frame of reference to measure and explain the Earth's motion. Thus, despite what most people think, there is no absolute "speed" or velocity of the Earth.
Q: "Doesn't this mean we'd be traveling faster than the speed of light, which is impossible?"
A: The equations of Special Relativity prevent an object from accelerating to the speed of light. Due to this restriction, these equations prove that an object can accelerate at a constant rate forever, and never reach the speed of light. For an in depth explanation: Special Relativity
The "Free-falling" Object
Assuming there is no drag, an object leaving the Earth's surface will undergo inertial motion until the accelerating Earth catches it. In our frame of reference, the object looks as if it is falling to the ground. Technically, in such reference frame, we can never tell whether the Earth is accelerating towards the object or the object is accelerating towards the Earth. However, one thing is certain: the object appears to be accelerating relative to us non-inertial observers.
In the FE universe, gravitation (not gravity) exists in other celestial bodies. The gravitational pull of the stars, for example, causes observable tidal effects on Earth.
Q: "Why does gravity vary with altitude?"
A: The moon and stars have a slight gravitational pull.
In the Round Earth model, terminal velocity happens when the acceleration due to gravity is equal to the acceleration due to drag. In the Flat Earth model, however, there are no balanced forces: terminal velocity happens when the upward acceleration of the person is equal to the upward acceleration of the Earth.
Q: "If gravity does not exist, how does terminal velocity work"?
A: When the acceleration of the person is equal to the acceleration of the Earth, the person has reached terminal velocity.
The lift force accelerates the plane up relative to the acceleration of the Earth. When the upward acceleration of the plane equals to the acceleration of the Earth, the plane can maintain altitude.
Q: "Isn't this version of gravity flawed? Wouldn't planes/helicopters/paragliders crash into the Earth as the Earth rises up to them?"
A: No. By the same argument, we could ask why planes/helicopters/paragliders don't crash into the Earth as they accelerate down towards it. The reason why planes do not crash is that their wings produce lift, which, when their rate of upward acceleration equals that of gravity's pull downwards, causes them to remain at a constant altitude.
The same thing happens if the Earth is moving up. The plane is accelerating upwards at the same rate as the Earth, which means the distance between them does not change. Therefore, the plane stays at the same height and does not crash.
Other Frequently Asked Questions
Q: "If the world was really flat, what would happen if you jump off the disc's edge?"
A1: In the general model, you would become directly affected by Dark Energy as the Earth is, creating the illusion that you are standing next to the Earth.
A2: In Mr. McIntyre's model, you would enter an inertial reference frame, traveling at a constant velocity relative to the Earth. In your point of view, it appears that you are falling into space.
Q: "If the Earth was indeed a flat disc, wouldn't the whole planet crunch up into itself and eventually transform into a ball?"
A: If the Earth generated a gravitational field, it would eventually happen. Of course, it would take a billion years for the process to complete. FE assumes that the Earth does not generate a gravitational field. What we know as 'gravity' is provided by the acceleration of the earth.
Q: "How is it that the Earth does not have a gravitational pull, but stars and the moon do?
A: This argument is a non-sequitur. You might as well ask, "How is it that snakes do not have legs, but dogs and cats do?" Snakes are not dogs or cats. The Earth is not a star or the moon. It does not follow that each must have exactly the properties of the others, and no more.