IDK how gravity or topology works.
In order for a planet to even be disk shaped gravity would have to function differently. But no if a planet was disk shaped with the current gravity rules, and something else was able to keep it in that shape against the force of gravity trying to round it back out, you wouldn’t uniformly feel the force of gravity at different points on the surface. Even the slight imperfections in the current round shape of the earth has an effect on surface gravity at different places.
that’s assuming Earth has uniform gravity, which is not the case either locally (due to changes in density of the composition of the earth) or due to earth bulging at the equator
The answer to your question is illustrated here in the first couple minutes of this old Vsauce video
Now, I’m confused. Does the magnitude of the gravity change or only the direction? Ooterness’s, linked physics analysis seem to show gravity would be super week near the edges. The video seems to suggest otherwise. Or maybe I’m just not understanding anything.
I’m no physicist, so I could be wrong, but I don’t think the magnitude would change, only the direction gravity pulls you in. Gravity would always want to pull you towards the center on disk-shaped planet. It might “feel” stronger the same way gravity might “feel” stronger when ascending an ever-steepening hill, though.
A planetary mass disc would collapse into a sphere. Gravity works towards the centre of mass, so it makes sense that it becomes a sphere once the force is great enough, where the distance from said centre of mass is nearly equal from all points on the surface. it’s the reason why smaller objects are usually oblong and weird shaped (e.g. asteroids) while larger objects are close to spherical (e.g. Pluto!)
And what do you mean by “uniform gravity”? If you mean for a theoretical person on the planet, it would not be uniform on the edges as you have an imbalance of forces towards either side, while in the centre it is symmetrical, an equal amount of mass on either side. On a perfectly spherical planet, it would all add up to a downward force on any point on the surface
One interesting note, some planets like Jupiter and Saturn spin so fast they bulge in the equator, making them a little less spherical than normal!
edit: All the planets spin and have this bulge, but Jupiter and Saturn have the most noticeable bulges of the Solar System planets due to spinning the quickest
Earth, too. By about 43 km.
A planetary mass disc would collapse into a sphere
… but galaxies are “flat”?
Not all galaxies are flat, and the stars in galaxies are sperated by enormous distances.
It’s the difference between Saturn and its rings. Saturn the plantet collapsed into a spheroid. The rings are pieces of stuff, not solid. Of all the stuff in the rings were thrown together into one solid clump, it would be a spheroid, too.
And I think we’re pretty lucky to see Saturn’s rings. Wait a few million years and they will collapse into a moon.
Yep, as are solar systems. But when they collapse in to single objects, those objects end up spherical once they’re about a third the size of our moon (or even less than that if they’re icy/gaseous).
In this case, it’s all to do with the conversation of angular momentum! This is also why the solar system is in a plane.
Basically, during formation, the cloud of gas forming the galaxy is spinning super fast, and the difference in rotational forces flattens the cloud closer to a plane.
A galaxy isn’t a planet. Planets themselves are rarely perfect spheres. Earth is actually an oblate spheroid. Solar systems also exhibit planar geometry. They all spin. The angular momentum is enough to overcome the gravitational pull. If that momentum was not enough to overcome gravity, all that mass would collapse into a spherical object. Depending on how much mass would determine what kind of celestial body it is.
Earth doesn’t have uniform gravity…
Are you calling me fat?!
Babe, you have a moon.
😆 Very nice. Thanks for the laugh.
It’s mostly uniform. There is variation by location, but the maximum difference is +/- 0.7% from the nominal value of 9.8 m/s^2.
The further you are from the center, the more skewed the gravity would be towards the center. I think that as the diameter approaches infinity while the thickness stays the same, that evens out a lot, but it will never become perfect near the edges.
That is to say, the flat planet will feel like a bowl, because the further out you are the more steeply sloped it is relative to the pull of gravity.
IIRC, the IAU’s definition of planet – infamously applied so that Pluto fell off the list of Solar System planets – requires that a candidate planet be large enough that its own gravity is strong enough to force it into a rough sphere, whatever it might be made of.
So a disc-shaped planet could not ever meet this criteria, because if it were made of something strong enough to remain a disk, then it’s too small to be a planet. And if it did exceed the critical size for gravity to make a sphere, then it wouldn’t be disc shaped anymore.
But setting that definitional quibble aside, we will focus on sizes and materials that allow a disk shape object to exist and be large enough for humans (or Mario) to visit. So no Wensleydale cheese. If we say that this object is mostly uniform in its mass distribution, then it would have to be the case that for any disc shape (including cylindrical), different points along the surface will be farther or closer to the center of gravity. Thus, inhabitants would experience gravity differently depending on where they are.
Note that we haven’t even considered whether the disc is rotating. If it is, then there’s a chance that the centrifugal acceleration at some points will completely negate the gravitational acceleration. At such points, one could hop up and off the surface, linger for a bit, and then get pulled back down once the disc has rotated to a position where there’s a net force upon you again. Or if the centrifugal acceleration is too strong, it might repel visitors on the surface altogether.
Alternatively, there would be a danger of playing on a trampoline that accidentally crosses into a net-zero gravity region. Here, a double bounce could send someone very high up, only to then plummet back down to their death when re-entering a downward gravity zone.
I have almost no citations for the above, but I thank you for posing an interesting question.
A disc-shaped body may not be a planet, technically. But it would still be a workd. A discworld, if you will. Ook.
Humans that successfully visit would be honored as a Discman.
Not the double bounce?!
Here’s the full physics analysis. Check out Figure V.4.
TL;DR: It varies a lot. If the average gravity over the whole disk is 1.0, it’ll be 2.0 near the center and taper off to ~0.2 near the outer edge.
Not a disc, but you may find this interesting: https://www.youtube.com/watch?v=fMlGs4X67q8
Every planetoid from about a third of the mass of the moon upwards is going to be ovoid rather than flat, because of the impact of its own gravity on its shape.
Center of mass would be at the center of the disc, so the further out you go, the more sideways will feel like “down”.
In other words, if you started at the center and started walking rimwards, it’d gradually start feeling like you’re walking uphill; Because relative to gravity you actually are. But the further out you go, gravity would also be weaker, as you’re further away from center of mass, so the “uphill climb” wouldn’t be that hard.
If the gravity is weaker overall, and also “down” becomes sideways, could you find yourself managing to do a higher than you’d normally achieve upward jump (relative to the ground, not the disc centre) and then end up falling for a very long time at great speed back towards the centre until you smash in to any geographical features?
