Flat World design

aramis erak

This video describes a potentially interesting gravitational effect on a flat earth. See 0:30 - 2:00. Seems closer you get to the edge, the "steeper" it seems.
Assuming, of course, that ...
  • gravity on the flatworld works as it does in the real world
  • there is an edge
If gravity is a function of mass and the square of distance, as in the real world, then yes, unless...
there is no edge. A perfectly flat plane in a closed figure universe has no center to angle to.

[edit: Snipped brain cramp moment... ]

Note that some fantasy games (officially including D&D when using Spelljammer setting) do NOT use mass-based gravity. Spelljammer's gravity rules are amusing and a bit insane... but can generally be teased out to coherent within classes...
Spheres: down is towards the center.
Platonic solids and dipyramids: down at/above surface is perpendicular to and towards the nearest face; inside, it's perpendicular to and away from the nearest face.
things with decks: perpendicular to the decks near the (IIRC) center of mass (but it might be center of volume).
Things without decks but rigid: along the long axis, towards center of length.
Things inside a larger thing's gravity zone do not experience/generate their own gravity plane.
Even humanoids generate a gravity plane... towards the waist. (If you experience incontinence when floating outside the ship's gravity plane, it flows up to your waistline, rather than down your leg)

It's one of those few areas where real physics is often best ignored.
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Just as an exercise, because I've thought about it off and on for years, and have never quite wrapped my head around it, I would love some input on designing a "flat world" campaign setting.
Done a lot of thought on worldbuilding. (So much so I've written a book on it.)
So happy to give some input.

1) the world has an ocean in the center, a ring of mountains on the exterior, and rivers that run down from the mountains to the ocean.
Sounds good so far, and solves the problem of water running over the edge. A "saucer" shaped world.

2) There is no sun, and no moon(s). Day is simply "when the sky gets light" and Night is the opposite. There are times of "nightglow" but there is no actual moon (unless we invent a way to have one).
This is where I start to question you. Gently.

As a rule, the more funky changes you make to the world, the more careful you have to become, because this can cause odd ripple effects.

Like how to werewolves work without a moon? Are there tides? Why is the spell moonbeam? How do magic items that recharge "daily each dawn" work when there is no literal dawn?
Would there be seasons if there is no sun? Do days still get warmer and hotter? Where is the source of said warmth?
Without a direct source of light, are there shadows? Which way do they fall?

This all seems trivial until you have a way of the shadow monk in a light forest asking which way the shadow of the trees are falling so they know where they can teleport.

3) burrowing through (or climbing over) the mountains in the outer ring would only end up "coming out the other side"; the dwarves probably make a lot of money creating travel short-cuts this way.
Do you mean they just emerge on the far side like the edges look around and connect? Like an older RPG video game map?

Questions: Seasons? What's "up there?" and "down below"?
Up in the sky? Down underground?
Or are you asking about the top side and bottom side of the saucer?

There could easily be stuff on both sides of the saucer. You could do traditional fantasy on one side, and non-traditional on the other.
Or have opposing factions with different races, like Warcraft, with an orc nation below. So elves and humans are only on the dorsal side while gnomes and dragonborn hail from the ventral.

What does the source of a river look like?
Like the real world.
Clouds are pushed up by air currents hitting the mountains, and cool in the upper atmosphere. Rain comes down and makes small brooks and streams that merge into larger rivers.

Are there other planes? Is there an underdark?
No reason there couldn't be other planes.

If the disc is thick enough, there could easily be an underdark.
The deepest real world mine is less than 4km thick, so a 10km wide crust would have plenty of room. (The highest building in the modern world is still less than a kilometer tall, so 10km gives you more than enough room for a dozen medieval cities stacked atop each other.)

Is there a "backside" to the world disc? Is there really even a "disc" if there's a wraparound effect?
See above for "backside" thoughts. No reason there couldn't be.

Is it a disc? Well, it's almost more the inside of a sphere. (See Hollow World). But since fantasy people might not be able to map everything, the disc/sphere question is effectively an armchair philosophical question for people in-world.
It doesn't matter in play for the most part and could just be a "mystery".

I think 1000 miles as a radius. That's ample, especially if there is also a full underdark layer or layers.
This is where I will direct you to Google Maps to actually find out how big 1000 miles is.

Because you just described a flat disc with a diameter the distance between Los Angeles and Detroit.
The "ocean" in the middle is really more of a large saline sea, possibly smaller than the Mediterranean.

And I have always liked the idea of the feywild/shadowfell as the ONLY alternate planes - a chaotic, natural world filled with elemental and fey creatures, and a shadow world filled with ethereal and undead creatures. None of the 4e mythology, though.
It's easy to leave the cosmology as vague and mysterious, being something only rare high level adventurers ever see.

But you might need to answer the question of "where do devils come from?"

2) the "sky" is an energy barrier of some sort, like a force shield. Within it, there are crystals that look like stars. Sometimes these crystals fall from the sky. They form something similar to a meteor, but when they land they usually cause either an explosion, a wave of magical corruption, or some other dangerous event.
Perhaps the crystals are continually growing and when they get to big, bits of them crack and fall off?

3) On the volcano island at the center of the world lives a VERY powerful being. It is worshipped/feared as a godlike being. It is called the Dragon Queen. there are around a dozen other dragonlike beings of great power and age who claim rulership over the world. Four of them have divided the Ocean (the world is a flat donut, with land surrounding the sea, and the volcano island in the center, and a wall of mountains at the edge) into territories, and 8 more have divided the land into their territories. I'm thinking these dragons feud constantly, warring with each other over who will be the next Mate of the Dragon Queen. Which happens on an irregular basis, every 30-100 years or so. Some sort of event occurs which marks the passing of the previous Mate, and at that point, whichever dragon-ruler is the most powerful leaves his/her realm and goes to the island. In their place, a new young dragon prince/princess emerges to take control of their realm. Which of course prompts all the other dragons to presume that this new one is weaker and less experienced, thus beginning a whole new era of unrest as they all attempt to gain power from the change.
Do the dragons have control over the people who live in their territory? Dragon overlords or emperors?
Do people measure time and eras based on the dominant dragons. Which ahem dragon age they are in?

But what is the History of this world? What do adventurers do? and why? Do they work for the dragons? Exist despite them? What do they plunder? Are there ruins, and if so where do they come from? I'm a bit stumped here...
This is where some history comes in. Deciding where monsters come from, where dungeons come from, etc.

If dragons used to rule over mortal kingdoms, they could easily have built up a civilization or two, which collapsed when that dragon became a "mate/meal" for the Dragon Queen. Destroyed by its neighbours. And then a new civilization emerged, was destroyed, and then replaced again.
Each region could be built upon the ruins of two or three previous civilizations, because every 30-100 years the ruler might leave and things fall apart.

A dozen dragon rulers. Assuming every dragon ruler's domain had its turn and an average turn around of 70 years, the oldest civilization might be 820 years old. Which seems ancient to someone in US but in England only takes us to King John, brother of Richard the First. (Yes, that King John.)
And that's not assuming a couple quick turn-arounds where a dragon is eaten every 35-40 years.

So you might have a really old kingdom that has managed to hang around for a thousand years, and is well established, but constantly afraid their time is coming. The young kingdoms that have been around for 50 years and everything is still just being rebuilt.

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