# Can density replace buoyancy?

Recently I discussed the possibility of a flat earth, with a flat earther whom rejected much of established science, especially newton and his laws because Newton's idea of gravity rendered a flat earth impossible (i assume that was the reason). One of his main points to justify flat earth theory was that gravity was false and that the densities of objects were entirely the cause of objects falling and rising on the earth's surface.

My main point of rebuttal was that there has to be a force for objects to change their course of motion ( ironically he agreed with newton's first law). And that if upthrust from the brownian motion of atmospheric gas particles was responsible for exerting a force UP towards the sky, then the same atmospheric gases that are above the object should exert an equal and opposite force down on the object, towards the earth assuming the object has the same surface area, facing the ground AND the sky; (so a a cube). Resulting in a net force of 0, i suggested that no object should be able to start falling OR rising at all from being stationary in these conditions and that pressure and area was more relevant than buoyancy. my question is, is my reasoning correct here? Am i misunderstanding the premise of the density vs gravity argument?

• You're wasting your time arguing with a flat-earther. Some people will fight you to remain ignorant because their favorite beliefs have become a religion to them. Commented Jun 22, 2019 at 2:14
• To note it, flat-Earth'ing is often a troll position. For the troll, it's sorta like a game where they try their best to argue that Earth is actually flat -- they may well find their own position completely absurd, but for them, that only makes it funnier.
– Nat
Commented Jun 22, 2019 at 4:57
• So I have this discussion many times lately. It seems as answered by a different person that they assume density and buoyance together replace the entire concept of gravity. This however then wouldn't make it possible for the moon to affect our tides for example. Also often they put in an incorrectly assumed religious aspect that the moon and sun orbit around us. Which has no actual value since an Orbit has no logic if earth is a floating disk. This is how you get into an endless back and forth about nonsense. Flat earthers don't want an answer. They want to spread misinformation. Commented Nov 17, 2023 at 8:25

If your friend believes in Newton's laws of motion and it is a logical person then he is doomed. Flat Earthers sadly are completely illogical in their aproach to reasoning.

Your argument is not correct and is a violation of Newton's 3rd law in fact. If Brownian motion of the gas particles is responsible of excerting the force that uplifts something the reaction force is not on the object but on the gas particles. You said that the gas acts on the object and then a reaction force happens also in the object that counteracts this influence but by the same argument there shouldn't be any kind of motion in the universe: if a push a box a reaction should appear that opposes my push. The thing is that Newton's third law establishes that the reacting force is acting in the object which excerts the action. Is not the same object. So if we assume brownian motion could be responsible for that then your friend is correct, a change in motion is expected.

The problem is that brownian motion can't excert any consistent push by definition. It is a random process of interactions. The overall motion would be zero since the same amount of gas particles is pushing the object in all directions on average.

Flat Earther's are also mistaken on the idea of bouyancy replacing gravity. Bouyancy IS itself gravity! An helium balloon "floats" because gravity is pulling both the ballon and the atmosphere to the ground but in doing so it is pulling the air with much more force than the balloon since it has more mass per cubic centimeter (which not only corroborates the existence of gravity as a phenomena but also the theory of gravity developed by Newton which states that the force is proportional to the mass of the object). Since the air is pulled to the ground more violently and since air is a gas and can fill any gaps it is occupying the volume where the balloon would be if there was no air at all: the ground.

This stupid idea that bouyancy is explained by relative density instead of gravity shows the lack of doubt in flat earthers. They don't question the idea of less dense objects going upwards and denser ones going downwards, they don't understand why this happens, they just assume that is like that (in general it happens) without any explanation because it was said by some authority (maybe at school). They don't understand that actual laws make that behaviour a consequence, instead they think of the consequence as a law.

The same stupid thing happens to them when they talk about sea level. "Water levels out". This is true in a wide variety of context for sure (just as the fact that denser things go downwards) but it is not "the law" it is just a consequence of applying the law in a specific context. In fact the law (which in this case is the law of gravity again) can go against this idea of water leveling in another context. Water levels because the gravitational field is uniform, but at large scales you come to realize that gravity is pulling to the center of earth and thus it is non-uniform (the direction of the force is different in different places relative to other), Water leveling is just a specific situation, a consequence of these laws, not the actual law you have to belief without any kind of skepticism.

• +1, however i did not intend to cite newtons third law. I thought that the object suspended in air would never have a resultant force on it much like how a cube does not accelerate to the left or the right when immersed in fluid as the fluid exerts the same force on the cube from both sides.. is this line of thinking also mistaken? thank you Commented Jun 22, 2019 at 17:48