Things that Would Happen if the Earth Was Actually Flat

The Top Ten
It would start to become a sphere immediately

That's just how gravity works. The planets are formed from the coalescence of interplanetary dust particles, whose trajectory is governed by the mutual force of gravity between them. This is a purely radial effect, acting towards the centre of mass of the ensemble at any given time; thus, the cumulative effect of this serves to create a spherical object.

Keeping a flat shape under gravity acting in three dimensions would mean the system would have to be spinning quickly to maintain a balancing centrifugal force. At such a speed, the planet would fragment and fly apart.

In three-dimensional space, a flat planet is simply not a stable configuration. Hypothetically, a toroidal (donut-shaped) planet is possible, but it too would have to be spinning rapidly to remain in this shape. A "day" on this planet would last two hours at most.

Gravity goes down. Down means towards more matter

Flat earthers have their claims debunked... severely.

People living on it would be pulled to the centre of the disk

Just like anything, the gravitational force you experience would act towards the centre of the object you're standing on. This would mean you would experience stronger gravity as you moved further from the centre, and the direction of this force would also become more horizontal as you moved further from the centre. Nearer the edge, there would be a stronger force pushing you sideways, or causing you to fall sideways, back to the centre of your flat Earth. Thus, you would never be able to travel to the edge.

Anyone living on it would suffocate

The way that the atmosphere would be pulled to the centre of the flat Earth would mean that the air would get thinner and thinner as you moved away from the centre. At the centre of the disk, the air pressure would be too high to breathe normally, at the edge, there'd be no air anyway. We'd all, at least in principle, have to live in a narrow ring where it is just right.
But that isn't all. We owe our existence on a spherical Earth to the dynamo effect of its core, which creates a magnetic field which shields the planet from harmful ionising solar radiation. For an Earth with no core, this protective field would be non-existent, and the solar winds would strip away the Earth's atmosphere, leaving virtually no air at all.

Welp, that'd be horrible.

There would be no concept of day and night

If the Sun were to illuminate a flat Earth, whether it be from a great distance as it is with spherical Earth, or as a much smaller object looping around the Earth as flat-Earthers tend to believe, it would be illuminating all parts of the surface people are supposedly living on at all times. Thus, we would be living in perpetual daylight.
If we wanted a day and night cycle, the Sun would have to be orbiting perpendicular to the Earth's surface, which wouldn't be a stable orbit. Even if it was, we wouldn't see the gradual transition from day to night that we do.

Much of the Earth would dry out

Due to gravity acting towards the centre, all the rain and snow would fall towards the centre and build up there, creating one large circular ocean, comprising most of the Earth's water. Further from the edge, one would receive less and less water. In fact, virtually none of the Earth's water would be on land, which would mean nothing could live there, and we'd have to rely entirely on sea plants for our oxygen.

Time zones wouldn't exist

However one may argue that the Sun behaves with respect to a flat Earth to make a day and night cycle, no configuration exists where both the cycle is stable and all parts of the Earth receive the same amount of daytime and nighttime consistently. If the Sun simply looped around the flat Earth, it would mean everyone gets constant daylight (I.e. no differences in time of day) and the two objects would eventually collide.

The moon would crash into Earth

Just as a flat planet cannot be maintained, an orbital system in which objects go in a simple circle around the flat object isn't feasible either, unless the distance between the two was so huge that we wouldn't see the moon at night. If the Earth became flat overnight, we'd all be dead pretty soon.

Seasons wouldn't exist

A planet gets its seasons from differing amounts of sunlight received over the course of a year. If there's nothing for the Earth to orbit around, or the Sun maintains a fixed distance from Earth, no seasons.

True, since there wouldn’t be any hemispheres. Also good to have you back.

Forests would grow at bizarre angles

Assuming trees could actually grow with all of this going on, due to negative gravitropism (I.e. growth against the direction of gravity), the trees would grow vertically as normal at the centre, but would begin to grow more horizontally as one goes further from the centre. Closer to the edge, the trees would be growing more and more sideways.

Lunar Eclipses wouldn't happen

Of course, this would just be a thin line shadow on the moon, assuming you could make this happen with these crazy orbits.

The Contenders
It would have no fluid core, and therefore no magnetic field

No magnetic field (along with the lack of many other dynamics derived from geo-magnetics) would leave the flat Earth unprotected from, at a minimum, the sterilizing properties of solar winds. The whole concept of a flat solar satellite is beyond ridiculous, its quite impossible. It could have never formed to begin with. Leave the weak and infirm to their own reasoning, the scientific process has no place there. Its as if a god were to argue with a grain of sand. Let them go, they are lost to us.

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