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This might be stupid, but is gravity a form of energy? And, if so, couldn't we use it for power?

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yes, we already do; en.wikipedia.org/wiki/Hydroelectricity –  lurscher Mar 17 '11 at 19:56
I don't think that something can be energy, it can have energy. Water power plants, that use waterfalls, use gravity that makes water fall. –  MBN Mar 17 '11 at 19:59
Amen to both of these comments! –  Ted Bunn Mar 17 '11 at 20:04
I had this thought when I was young. The answer that made me start to change my thinking was that gravity is a "conservative field". en.wikipedia.org/wiki/Conservative_force –  AlanSE Oct 25 '11 at 14:01
they say that gravity is not a form of energy,but the gravity of the moon pulls millions of tons of water every every 24 hours around the earth that can be used as energy and this is going on for millions of years.Is that not energy created from gravity? –  user14493 Oct 29 '12 at 6:41

8 Answers 8

up vote 6 down vote accepted

Gravitation is something that gives rise to a force. Like most forces, it can be put to work, just like in dams.

But gravitation is not energy: it's an interaction between physical bodies with mass.

NB: presence of mass warps space which affects the way massless things like light propagate. That does not mean that light is affected by gravitation directly. Only indirectly.

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Good answer but this couldn't really serve as a definition (or at least description) of gravity which also affects massless objects, e.g. light. –  Marek Mar 17 '11 at 20:39
You use the terms "gravitation" and "gravity" synonymously. Your answer would be clearer and more internally consistent if you used "gravitation" instead of "gravity" because the latter conjures mental images of a material substance in students' minds. –  user11266 Dec 7 '12 at 15:54
If I get your point right, @JoeH, you're saying that gravitation (a verb) is more suited to the correct interpretation as an interaction, whereas gravity (a noun) tends to be picked up as a thing? –  adavid Dec 12 '12 at 20:45
YES! YES! Absolutely! This is one of the most common classroom misconceptions, especially in introductory courses. Unfortunately, when I attempt to illustrate that here I get down voted. LOL –  user11266 Dec 12 '12 at 22:41

Gravity is a FORCE (not ENERGY) that is very similar to that of mechanical spring and magnet!!! Here, we shall compare gravitational force with force exerted by mechanical spring as magnetic force can also be generated via electrical current.

Work has to be done on an object to lift it up and thus gaining potential energy. It is very much like work being done on a mechanical spring to store the energy therein. When an object is dropped from higher ground, it is very much like the stored energy being released from a loaded spring converting the stored energy to kinetic energy.

Both Gravity and mechanical spring are not capable of performing work on their own. It is the stored energies that perform the work.

Note: In the case of waterfall as discussed here by others, the water is being lifted up by other form of energy such as the heat from the Sun and other form of energy from the Earth itself.

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Gravity, $\vec{g}$, as discussed in introductory classical physics is actually the gravitational field. Fields are fundamental entities that carry momentum and energy.

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Well, that's the question: do they? We can track the energy of the electrostatic field via E^2 dV, and when field energy disappears it reappears as kinetic energy of the charged particles. Gravity is different. If we calculate the square of the gravitational field and integrate it over space, we get something that looks like the electric field energy. But if we drop a stone to the ground, the kinetic energy increases and at the same time, the square of the field energy also increases. So you can't keep track of energy changes by looking at the field. It's very different from electrostatics. –  Marty Green Oct 25 '11 at 23:21

Gravity is not a form of energy but gravity creates potential energy. The way we use dams is by expoliting the force of gravity on a object that has potenial energy; which as the water falls it loses potential energy and gains kenetic enrgy proprtionatly. So yes we do use gravity to create energy but the reason we can use it is because the earth formed weird geographical formations that we use to exploit gracity. Moving an objects like water to create energy takes more energy then you get from it due to friction and other forces. (Gravity effects light because lights Wave–particle duality.)-as far as I know.

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Surely Gravity doesn't create energy as energy cannot be created or destroyed but only change state, surely when we say gravity has an effect on something it is energy changing state from whatever gravity's energy is, or whatever the energy state of what we call gravity is, to kinetic energy?

So from your above example Water at rest has no energy (other than the chemical energy that in this example doesn't change state) The "effect" of gravity is actually gravitational energy changing state to kinetic energy when the water starts moving, then when the water meets the dam the kinetic energy, remembering the 3rd law of motion, changes state through friction, vibration, noise etc. The water at rest again in the dam is still under the effect of gravity, or gravitational energy is changing state to kinetic energy when it flows over the dam, for instance which, if we put a turbine in the way would transfer it's energy from the water to the propeller blades which turns a drive shaft which turns a turbine, which changes ultimately kinetic energy into electrical energy.

I could well be wrong, and probably am, and would appreciate if anyone has a different view.

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Gravity is a form of energy. If there a gravitational field, then it has energy. There are well defined rules in Einstein's theory to calculate the energy per unit volume in a gravitational field. Its usually a very very tiny number, but the extreme example is a black hole, made of nothing but gravity, it has a huge energy density.

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Gravity is a force that is universal. How the force is linked to mass I do not know nor does anyone else. But, we view the solar system and the universe that we can and see that based on our estimations of size, density, and mass the universe seems to comply with the mathematical equations of Isaac Newton on a macro level. Once Einstein came along and we had the technology to view things invisible to the human eye the initial Laws of Newton were improved by Einstein. Is there any instance where gravity is not linked or dependent on mass? I think not. When we see the bending of light due to extreme gravitational fields we can only guess (if you want to sound scientific you can also say "Form a hypothesis") as to why. You might even extrapolate that all forms of energy would also be "bent" by extreme gravitational fields. After all: that is in the new "Star Trek" movie! So I am going with the old answer based on Newton's calculations; "The gravitational force between two masses m1 and m2 is given by the relationship:" http://hyperphysics.phy-astr.gsu.edu/hbase/grav.html#grav

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Gravity is Energy. Think of it like this, take an apple which has say a gravity of one it has this gravity in the center of it cut the apple in half and the gravity divides itself between the two halves and each apple has 1/2 gravity and each half apple has its own center of gravity keep on cutting and you will keep on dividing the gravity till you end up with I do not know what now blow up the thing that i do not now and it will take I think less to no energy to blow it up. why I say that Gravity is Energy is because how can something always change its center every time the matter is cut in half "think about it". this is the energy that can not be created nor destroyed. we will never see gravitational waves because gravity is an inverse energy without it we will have a static universe. If you are not convinced That Gravity is energy forget about potential energy for a while imagine that there is no bottom to the earth and you fall continuously forever being pulled by gravity you have wind turbines hooked up to your ship you will forever be converting gravity to Electrical energy its gravity that got the sun to burn in the first place compressing the atoms together and vwala ignition potential energy is there because we have an end point the ground, to atoms colliding its only there because that is what makes sense for us pull the rug and all hell breaks out. Gravity is energy take a clock that operates with weights into space were there is no gravity that clock will not function that is an impossibility in space were there is no gravity to operate the clock.

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I didn't down-vote you. First few sentences are somewhat acceptable. But, the larger part of your answer contributes nothing as regards the question... –  Waffle's Crazy Peanut Oct 30 '12 at 11:43

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