## Free Fall

Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. For a demonstration, we can ask any flat-Earthers the following simple physics problem:

“A rigid object is released at the height h above the ground, what is the object’s velocity right before it hits the ground?”

Flat-Earthers will not be able to solve this free-fall problem without using the gravitational acceleration g=9.8 m/s². Some will call it by a different name, showing that it is just a “branding” problem. Practically all the time, they will not involve density to solve it, if they can solve it at all.

## Archimedes’ Principle and Gravity

Archimedes’ principle states that any object, totally or partially immersed in a fluid, is buoyed up by force equal to the weight of the fluid displaced by the object. Meanwhile, Newton’s law of universal gravitation states that every particle attracts each other with force directly proportional to their masses & inversely proportional to the square of their distances.

Flat-Earthers like to characterize gravity as if it was “invented” to “replace” Archimedes’ principle & that the two are competing theories. In reality, these are two different theories that explain different phenomena. Gravity does not explain anything that Archimedes’ principle explains and vice versa. Gravity does not replace Archimedes’ principle; both are valid and in use today.

## Countering Gravity

Gravity is not the only force. Other forms of force exist. Multiple forces can affect an object at the same time. Forces can counteract gravity, making an object move against the direction of gravity. These cases do not disprove gravity.

Objects can move against the direction of gravity, and flat-Earthers use it as “evidence” that gravity does not exist. In reality, there is at least a force other than gravity affecting the object, in the opposite direction from gravity, with a larger magnitude. Continue reading “Countering Gravity”

## Density and Balance Experiment

Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. We can disprove flat-Earthers’ claim by setting up two different weights of the same material (thus, the same density) against each other using a balance. We can see the objects’ densities are not why they fall down.

## Archimedes and the Surface of any Fluid

In the 3rd century BCE, Archimedes of Syracuse wrote what we now call the Archimedes’ principle in his book “On Floating Bodies” using the spherical Earth model.

Flat-Earthers like to misuse Archimedes’ principle as if it supports their claim that Earth is flat. In particular, they abuse Archimedes’ principle as if it is a competing explanation against gravity. In reality, Archimedes and other Greek scientists at the time already knew Earth is a sphere, and he explicitly mentioned it in his writings.

## Air Density vs Object’s Weight

At a lower altitude, the air is denser, and objects weigh more. On the other hand, at a higher altitude, the air is less dense, and objects weigh less.

Flat-Earthers deny gravity and invented the “explanation” that an object falls only because it is denser than the air surrounding it. But if it were true, objects should weigh more at a higher altitude where the air is less dense. But they weigh less instead, disproving flat-Earthers’ density theory.

## Gravitational Acceleration in Archimedes’ Formula

Archimedes’ principle states that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces. Today we usually use B =  -ρgV to calculate the buoyant force, where ρ is the fluid’s density, g is the gravitational acceleration, and V is the volume of the displaced fluid.

Archimedes discovered buoyancy earlier than Newton discovered gravity, and flat-Earthers dispute the presence of g in the buoyancy formula. In reality, buoyancy depends on the weight of the fluid, and the distinction between weight and mass only occurred after Newton. Archimedes’ principle still applies, only that we now have a better understanding of what weight is.

## Density Tower

A density tower is formed by layering immiscible liquids of different densities on top of each other. It is possible because Earth’s gravitational acceleration pulls denser fluid particles stronger and displace less dense ones upward.

Flat-Earthers claim that the possibility to have density towers disproves gravity. In reality, a density tower cannot form without the presence of acceleration, like Earth’s gravitational acceleration.

## Egg and Salt Water

An egg will generally sink in freshwater, but if we pour salt into the water and stir it, then the egg will start to float. The reason is that after salt is added, the density of water increases. However, the relationship between density and if the egg floats or sinks is not a direct causality.

Flat-Earthers claim that the phenomenon “proves” things fall because of density, and that gravity does not exist. In reality, the density of the water determines the magnitude of buoyancy the egg receives. If the buoyancy is higher than the weight of the egg, the egg will float, and vice versa.

## Buoyancy

Buoyancy is an upward force exerted by a fluid (liquid or gas) that opposes the weight of an immersed object. Buoyancy happens because the fluid has a pressure gradient. Pressure gradient occurs because the fluid is affected by acceleration, such as the Earth’s gravitational acceleration.

Flat-Earthers makes buoyancy as an “explanations” on how things fall. They are wrong. Without Earth’s gravitational acceleration, buoyancy will not occur.

## Hot Air Balloon and the Force of Gravity

A hot air balloon rises because it has buoyancy. Buoyancy is the upward force that is exerted on the trapped hot air because its density is lower than the surrounding air. If buoyancy is greater than the force of gravity — or the weight of the balloon—, then the balloon will rise.

The basic physics escape most flat-Earthers. To them, the fact that a hot-air balloon rises is an ‘evidence’ that gravity does not exist. They believe things go up or down because of density. They are wrong.