What do flat Earthers think causes the "magical downward force"?

They don’t know - and not all of them agree that there is a “magical downward force”.

If (3) was the case - you’d have the same problems as (1) - but this weir...

3. Gravity doesn’t exist - it’s a form of magnetism that works on non-metals.

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4. Gravity doesn’t exist - things are held down by air pressure.

If (1) was the case - the center of gravity of the Earth would have to be so deep down to avoid gravity pointing off at an angle when moving towards the “rim” of the flat earth that the difference in height of a mountain would be negligible and we would get very, very nearly the same gravity everywhere.

1. Gravity is real - but the Earth’s disk is very thick so the force is very nearly vertical.

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I have heard them say:

2. Gravity does not exist - the Earth is accelerating upwards at 9.8 m/s2 which seems like gravity.

If (2) was the case - then the acceleration would have to be EXACTLY the same everywhere - and it’s not.

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…and there are probably other ideas out there.

The trouble with (1), (2) and (3) is that these cannot explain why the force of gravity varies with height above sea level…less at the tops of mountains - more in valleys.