Most egregious Flat Earth claim - Gravity is simply buoyancy & density - Debunked
First of all, buoyancy depends on gravity. If there is no gravity, there can be no such phenomenon as buoyancy. Look up the definition of buoyancy on Wikipedia or search buoyancy NASA on google, if you don’t believe it.
If gravity is simply buoyancy and density then why does hot air rise and not go down instead, or sideways? I’ll tell you why. Because we live in an environment where there is a downward force acting on things based on their mass. This can be experimentally verified and it does not depend on assuming the earth is a sphere. If I put 5 apples in a grocery scale I will get a certain weight (force). If I put an additional 5 apples of about the same size in the scale, the scale will register about twice the weight. So the force known as buoyancy depends on this downward force proportional to mass. This is why the deeper you go in a body of water the more pressure there is. This is why the higher you go in the atmosphere the less pressure there is.
Liquid water is nearly incompressible, so its density does not change much as you go deeper. In contrast, the atmosphere is made up of a mixture of gases, and they are very much compressible. So as you go up in the atmosphere, with the decreasing pressure the gaseous mixture becomes thinner or less dense because it is becoming less compressed.
If a mass of air were of the same temperature as the surrounding air, it would not tend to go up or down, because the upward forces around it based on decreasing pressure with altitude are balanced by the downward forces upon it based up its mass. But if the air were hotter than its surrounding air, its density would be less so that the downward force based on its mass would be less than the upward force due to decreasing pressure with altitude so the resulting balance of forces would cause it to rise. This is how a hot air balloon works.
Interestingly, forces greater than 1 g (i.e. 1 earth’s gravity) acting upon masses and proportional to mass can be artificially produced mechanically with centrifuges, or in an airplane in a steep coordinated turn, where a very steep turn may be alternately referred to as a high g turn, or pulling many g’s.