Where does gravity get it's energy from?

[londonbarry]

I can understand that most the energy on the earth can be traced back to the energy generated from the sun, but what about the energy which causes the tides, i.e. the gravity of the moon and sun. Where does that energy come from?

[QPR]

It's generated by the rotation of the planets/moon!

[londonbarry]

That can't be right. Are you saying that something that is not rotating does not experience gravity? Also, the force involved in something that is rotating is centripetal force, which is the exact opposite of gravity.

[tomd]

Every body has a gravitational force proportional to their mass, it's because the earth's mass is so large the force is noticable. http://science.howstuffworks.com/question232.htm

[londonbarry]

Ok, I follow that, but is it a possiblity that in a hypothetical solar system that has a lifeless sun, and also orbiting planets that it would be possible to generate electricity, on one of these worlds that also has ocean, from tidal action alone?

[LeMarchand]

I don't think that it's got anything to do with rotation; while SF space stations have rotating sections (if they are trying to be accurate), the gravitational effect thus caused is an application of centrifugal force: "down" is the outside of the spinning cylinder. As stated by tomd, gravity is proportional to mass. The best depiction of this I have seen was when a scientist took a spongy mat and placed various spherical objects on it. The greater the mass of the object, the deeper the gravity well around it. A marble rolled along the mat would tend to fall into the dip caused by one of the "planets". The solar system has balanced itself out over the eons so the Moon doesn't crash into the Earth, but it's gravitational pull does effect the tides. I guess the energy is the potential energy (my physics is a little rusty) from the difference between the two gravitational pulls. Hope this makes sense!

[norfolk boy]

Tell me i'm wrong but i thought the earths gravity came from the fact the earths crust was magnitized ?

[Drewhound]

OK, you are wrong lol. They are two different things, you can pick something made of iron up with a magnet using the magnetic force generated but not using gravity which is a product of the mass unless that mass exceeds the mass of the earth, gravity being proportional to mass Also the magnetic centres of the earth are nowhere near the physical centre and are always on the move

[RickyB]

The changing tides only exist because of (effectively) 3 things; 1. The rotation of the Earth. 2. The orbital motion of the moon around the Earth. 3. The orbital motion of the Earth around the Sun. You are correct that the energy must come from somewhere - and that is predominantly from the rotation of the Earth. As a result - the Earth's rotation is slowing down - and eventually, the Earth's rotation will be the same as the orbital period of the Moon. When that happens - the tides will then only be due to the Sun - the Moon will remain overhead at roughly the same point above the Earth - so the tides will not change due to the Moon. Because there will still be tides due to the Sun - this will "steal" energy from the Earth and Moon - causing the Moon to gradually draw closer to the Earth - and eventually it'll get close enough that it'll likely be destroyed by the Earth's gravity - but don't worry too much - it could take several tens (or maybe hundreds) of billions of years yet - long after the Sun will have died off. Due to the fact that the high tide does not quite occur directly above the Moon - the Moon is currently being accelerated by the Earth's tides and is getting further away from Earth at the moment - it's about a foot further away than it was at the time of the Apollo missions.

[lobster]

Well there is a load of study into sub atomic particles and anti-matter. one possibility for why there is a force of gravity is the theoretical existance of graviton particles. these are thought to possibly generate gravity energy. the theories outlined above would therefore become examples of how gravitons affect systems. the future of physics is sub-atomic, a realm which is largely beyond imagination as there is no earthly situation to describe the theory.

[New Forester]

There is a bit of confusion here between force and energy. Gravity is not energy, it is a force. Energy is released when objects move nearer to each other, or slow down. RickyB's explanation is good. The water moved by the tides is very heavy, but very very small compared with the mass of the solar system. Some of the energy in the earth-moon-sun gravitational system is converted from their movement to movement of water. The earth, moon and sun all slow down a tiny bit each tide. Eventually as RickyB says it does all add up. It is tides in the ocean and rock crusts which have stopped the moon rotating more than once a month, so it now turns the same side to the earth all the time. Likewise the earth used to rotate faster, giving more, shorter days in the year.