Slowing of Earth's Spin

[Fishteacher73]

This though arose on the news section with the generating bouy. It was expressed that this would deplete the gravitaionally enduced momentum in out spin and eventually slow the Earth.
I disagree with this scenerio (Although it is true the Earth is slowing. Between 1 -2.2 seconds per 100,000 years). As opposed to have a discussion on the comment section of the news post I decided to make a thread about it.

[UncleAl]

gravitaionally enduced momentum in out spin

Ugh. Gravity has nothing to do with it. The Earth's rotational energy is 2.137x10^29 joules. If you pull some out the Earth's spin slows. 2002 worldwide primary energy production was 4.3x10^20 J.

Most of the Earth's mass is in its Mars-sized iron core. That is exactly the wrong mass distribution for an efficient flywheel. Even minor mass perturbations alter the Earth' spin rate. The recent Richter 9.3 Indonesian temblor knocked a microsecond off the day via mass redistribution. China's Three Gorges dam will pile up 1.39 trillion cubic feet of water (39.4 billion tonnes), adding microseconds to the day.

Generating power from tide, wind, and wave is incredibly stupid. It is the Enviro-whiner trinity of trading nugatory fears for real world disasters: expensive, shoddy, deadly.

[bumab]

UA is right about the mass distribution being the issue. It's really just a conservation of angular momentum problem, you redistribute the water, you affect the mass distribution on the earth and spin has to change to accomodate. Same with affecting the tides and the moon- affect the tides (i.e. utilize them for energy production- hence slowing them) you affect the moon.

However, the assertation that it will be the end of the world is a little dubious. The overall effect of this slight perturbation is so small, it is really negligable compared to other methods of power production. I say go for it.

[C1ay]

Originally Posted by bumab

However, the assertation that it will be the end of the world is a little dubious. The overall effect of this slight perturbation is so small, it is really negligable compared to other methods of power production. I say go for it.

Yeah, I'm not completely sold on that either. I think a lot of the energy in the oceans is indirect solar energy that is a result of the sun heating the oceans. Converting that energy to other forms is energy that the sun will continue to contribute to the Earth's energy total. These buoys will extract energy from the Earth but will it be more than the energy the sun imparts on the Earth's oceans?

[bumab]

It's not that you are somehow extracting energy from the moon's gravity in a subjective, ephemeral way. The tides pull on the moon just as much as the moon pulls on the ocean (causing the tides). Slowing those tides to create power will affect the moon in the same way. I think Uncle Al explained it pretty well in the actual news thread.

It's not about how much energy you extract, but what you are affecting by extracting it. Everything effects everything. You gain energy from falling water, but you also slow the water down. Same thing with the moon. You gain energy from the moon pulling on water, but you also reduce the amount of pulling the water can do on the moon (by redistribution of the water's mass), and thus increase the moon's orbit.

Clear as mud?

[Smokinjoe9]

I know this is of base but, Is it thought the Earth's spin assists gravity, or is generated by gravity? Or better yet has nothing to do with it.

[Fishteacher73]

Gravity is a property of mass not motion. Through inertia forces can be exerted that seem like gravity, but are not resultant from.

[Smokinjoe9]

Originally Posted by Fishteacher73

Gravity is a property of mass not motion.

Relative to mass?

[Fishteacher73]

Originally Posted by Wiki

Gravitation is the tendency of masses to move toward each other. Weight is a force caused by gravity equal to the product of the acceleration of gravity and the mass of the object. Exactly why two masses separated in space have a gravitational attraction to one another remains largely unknown, despite much research and various theories.

As per Wiki.

The force of attraction = the product of the two masses devided by the distance multipled by a gravitational constant.
F=(m1xm2/r)G

[C1ay]

Originally Posted by bumab

It's not that you are somehow extracting energy from the moon's gravity in a subjective, ephemeral way. The tides pull on the moon just as much as the moon pulls on the ocean (causing the tides). Slowing those tides to create power will affect the moon in the same way. I think Uncle Al explained it pretty well in the actual news thread.

It's not about how much energy you extract, but what you are affecting by extracting it. Everything effects everything. You gain energy from falling water, but you also slow the water down. Same thing with the moon. You gain energy from the moon pulling on water, but you also reduce the amount of pulling the water can do on the moon (by redistribution of the water's mass), and thus increase the moon's orbit.

Clear as mud?

Yes, if you do something to damp the tides it extracts energy from the Earth-Moon system and you do have an effect on the orbital radius of the moon.

The oceans are not limited to tidal energy though. In addition to the approximate 12 hour cycle of the tides there are other waves moving up and down in ocean that derive their energy from the weather. Winds moving across the surface of the water impart energy into the oceans as does the heat of the sun. This is all of the smaller, choppy wave activity on the surface indepent of the long wavelength activity of the tides. This lower amplitude wave energy riding on top of the tidal wave energy should be available for extraction.