Density and buoyancy
Gravity is incompatible with a flat Earth,[1][2] so flerfs must find some other way to explain why massive objects fall and exibit weight. Many of them claim that "density and buoyancy" are a sufficient explanation. According to flat Earthers, objects magically know which direction to move through a medium based on their relative densities. It should be noted that density is a vector-less quantity, meaning it cannot cause motion in any direction.
As of 2026, no flat Earthers have been able to successfully derive the downward acceleration at Earth's surface of 9.8 from the physics of density and buoyancy.
Background
Eric Dubay's "proof 115" from his book provides a summary of the flat Earth argument:
The existing laws of density and buoyancy perfectly explained the physics of falling objects long before knighted Freemason “Sir” Isaac Newton bestowed his theory of “gravity” upon the world. It is a fact that objects placed in denser mediums rise up while objects placed in less dense mediums sink down...
Apparently, the mere observation of denser objects sinking is in itself an explanation of the observation. This is begging the question, an informal fallacy that flerfs use quite often.
Some flat Earthers such as David Weiss believe that density and buoyancy can still work if you first "establish a down" with electrostatics, as he explained in this interaction:
Everything goes down because of buoyancy and density. This is what most flat Earthers say... I'm with it 99%... but I'm like hold on, what establishes a down? The Earth has electricity in it. It has a static charge to it, and that can be measured scientifically. The higher you go the higher the voltage goes. So that establishes a down, and buoyancy and density sort everything out.
Density
Density is a physical quantity describing the amount of mass per unit of volume. The SI unit of density is .
Neither mass nor volume are vector quantities, so neither is density. This means it cannot cause motion whatsoever. It can only influence how an existing, external force interacts with the object.
If "down" is defined by the electrostatic charge of the ground, then density becomes irrelevant. The force is determined by the net charge of the object, and the acceleration it experiences is found by dividing this force by the mass.
Thought experiment
The flerf claim is that density is responsible for weight. Imagine a balance scale with a marble on one end and a container ship on the other. Which end hangs lower? Keep in mind the container ship is less dense; it floats on water while the marble sinks. If density were really the sole cause of weight, you would expect the side with the marble to hang lower.
Buoyancy
Buoyancy is a force exerted on an object displacing a fluid by that fluid. It acts in the opposite direction to the object's weight force (i.e., upwards). The formula for buoyant force at Earth's surface is (density × displaced volume × gravity of Earth). in the formula can refer to either gravitational field strength (expressed in N/kg) or acceleration due to gravity (expressed in m/s2).
Gravity is required for a buoyant force to be created in the first place. A fantastic confirmation of the First Law of Flerf!