What would be needed to create a gravity-mill?

Wouldn’t work without adding energy to the system. What you are describing is a perpetual motion machine which is not possible.

@Caravanfan what a delicious comment.

But, could you tell me if having the impossible cog that has no substance in reality would help? If we have the paddle part to catch the gravity, can it swing back around if it is pinned to an imaginary hub that exists only as an idea? Or, does the paddle simply point straight down even with a zero weight/resistance wheel/hub fantasy assembly?

You’re talking about impossible things here. They don’t exist in reality. The first and second law of thermodynamics proves that perpetual motion is impossible, even the universe will cease to move one day.

@Dig_Dug and I are saying the same things. There is no such thing as zero resistance. In reality it won’t work. Sure, if you want to make up a perpetual motion machine in fantasy you can do it. You can also say pigs fly.

Perpetual motion machine (batteries not included).

Also relevant: Will a Levitating Gyroscope Spin Forever in a Vacuum Chamber?

@gorillapaws lol. Good one!

You can actually use the Moon’s gravity to generate power though. It’s called tidal power, so in a way that’s very much a “gravity mill” of sorts.

@gorillapaws But it’s not infinite, obviously. And when we do use tidal power we are slowly decreasing the moon’s period of rotation.

Earth has been flinging the Moon further and further out since it was created. Earth’s 24 hour rotation has been increasing the Moon’s period the whole time due to the eccentric gravity of Earth pulling the Moon around faster than it’s original speed. This has caused the orbit to gradually increase. I suppose using tidal power on Earth would slightly increase the gravitational eccentricity and cause the Earth to pull slightly harder on the Moon, and that will slightly increase the acceleration of the Moon’s period.

Wikipedia – ” Energy Vault is a Swiss-based, global energy storage company specializing in gravity and kinetic energy based, long-duration energy storage products. Energy Vault’s primary product is a gravity battery using a multi-headed crane to store energy by stacking heavy blocks made of composite material into a tower, capturing potential energy in the elevation gain of the blocks. When demand for electricity is high, the crane lowers these blocks to the ground, with the motors functioning as generators and delivering electricity to the grid.”

Wikipedia – ” Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost surplus off-peak electric power is typically used to run the pumps. During periods of high electrical demand, the stored water is released through turbines to produce electric power. Although the losses of the pumping process make the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand, when electricity prices are highest.”