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Can Gravity Make Energy?

Shadow said:
Has anyone heard of any known hypothetical ways to extract energy from gravity without resorting to fusion and massive bodies of matter.
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if you reject a good ol'water wheel, then i expect you cannot avoid "resorting to fusion", you cannot make rocks or (probably) any of that other neat stuff, including the oxygen in water, without fusion.

(stardust).
 
We use gravity all the time to get energy, we simply slow down the rotation of a planet and use that energy to make our space ships go faster. Of course it's not free energy, as eventually we make the planet stop spinning altogether, so it's really just a much bigger scale of the mountain thing, but we can do it. So why not shoot a few space ships at mars, speed them up really lots, then crash them into something, letting off heat which we then use to heat water and drive a turbine? Done and done. Gravity into usuable energy. It's time for smug mode.
 
That's using gravity to move kinetic energy between one body and another. Also, I think you'll find its the orbital motion of the planet that gets leached, not its axial spin.
 
Terry said:
That's using gravity to move kinetic energy between one body and another. Also, I think you'll find its the orbital motion of the planet that gets leached, not its axial spin.
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Are you trying to make me look stupid? :)

Well I have attended several astronomy lectures, just for lame interested citizens like me, and I could have sworn they always said the spinning, but it may have just been dumbed down or I am mis-remembering. I may look it up if gets slow at work.

However, (you didn't think I would just let my method die so easily did you?) I still maintain it's the best way we have to extract energy from gravity. Sure technically we aren't extracting it from gravity itself, but I think it's the closest we can get with current technology, and maybe ever. And who really cares if mars or jupiter stops spinning? We should just make sure the big red spot is pointing at us when jupiter does stop, for better views from earth.
 
Terry said:
That's using gravity to move kinetic energy between one body and another. Also, I think you'll find its the orbital motion of the planet that gets leached, not its axial spin.
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Why can't it be both? The "slingshot" thingy we do with deep space probes steals a bit of orbital motion. Launching a satellite with a space elevator would steal some axial spin, but that wouldn't be stealing from gravity, strictly speaking... (Disclaimer: Based on handwaving physics.)
 
I think it would be wise at this point to differentiate between using gravity as a means to extract energy from some system as opposed to obtaining energy directly from the gravitational field itself. Many celestial bodies are coupled together via gravity -- the Earth/Moon pair being a good local example. When one extracts energy from using tidal motion (as mentioned earlier) they are obtaining energy from the kinetic energy of the Earth/Moon system; this in turn slows down their motion. The problem also lies in how we define energy (which is the same as work) in units -- it's Force x Distance or Power x Time as just 2 examples. Gravity is not really a force by itself -- it's an acceleration. (Unfortunately common use of the word Gravity makes people think it's a force -- but technically it's not.) To get energy out of it one needs a mass and some distance over which the force is applied. This force is commonly known as the mass' weight. Without gravity, the mass is just ... well, mass. Put it in a gravitational field and now it has weight. Let it fall and now you have energy -- as in a waterfall.

Basically you can't get energy out of something that isn't a form of energy to begin with.
 
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Huntsman said:
Um,

Gravity is a force.
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Units please ... for gravity?

You see, even among the scientific community, gravity is called a force -- but when used in equations to determine things such as weight, energy, power, work, etc., it clearly is just an acceleration. Since we are speaking here of getting energy from just a gravitational field, we need to look at gravity in its mathematical application, an acceleration. The basic equation of F = ma or W = mg clearly shows that mass is needed to produce the force (weight). Alone, g, is not a force. Please go here to see the terms used back and forth.
 
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Just thinking said:
...
In the sense we need to determine if energy can be extracted from gravity we must treat it as an acceleration used to produce the force (weight) which when applied over a distance yields energy.
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Not really. :)
 
F = G[(M1*M2)/r^2]

We use acceleration because it's less computationally intense, but your argument amounts to the same thing as declaring electromagnetism to be an acceleration instead of a force. You use a similar equation to determine attraction between to oppositely charged particles, for instance.

The acceleration inparted is determined by the value of the force between them. Since it is relatively easy to determine acceleration experimentally, and (when dealing with large bodies, such as the case of weight) acceleration is pretty well independent of mass, we use acceleration.

Howewver, your view only applies when the acceleration produced is already known. Say there's a planet with a mass or 100,000,000,000 kg. What acceleration would you use to determine the gravitational attraction between it and a 10,000 kg spaceship? You have to figure the Force first, to get to the acceleration (or, at a minimum, work the force equation into the F=ma equation).
 
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Just thinking said:
Please explain.
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Whether you use the name 'gravity' for the force of gravity or the acceleration of gravity, the result should be exactly the same. If not, it is no longer a semantics debate, like "the basic equation g=F/m, clearly shows that mass is needed to produce the acceleration"...

Huntsman's (OK, Newton's) more general equation doesn't even include an acceleration, since it is not a basic constant, just a convenient parameter to use close to the earth surface.

[latex]$$
F = G\frac{m_1 m_2}{r^2}
$$[/latex]

Once you get that stuff, talk to Albert... ;)
 
Working from Huntsman, setting m1 = m and g = a, thanks to Newton, we get

[latex]$$
F = G\frac{m_1 m_2}{r^2} = ma

a = G{m_2}/{r^2}

a = g = 9.81{m}/{s^2}

$$[/latex]

Therefore, on earth, a free 1 kg body is subjected to a gravitational force of 9.81 N, and experiences the acceleration due to that force, g.

I didn't realize there was actually a debate! :confused:

edit: gngngn stupid formatting... little help?
 
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Ririon said:
[latex]$$
F = G\frac{m_1 m_2}{r^2}
$$[/latex]

Once you get that stuff, talk to Albert... ;)
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That is nothing new ... it is the force of attraction between two bodies (in this case using gravity) -- it can easily be re-written to explain electro-static attraction using charge instead of mass, and k instead of G -- making Coulomb's Law. The key point in either one is that two masses or charges are needed to produce a force.

Let's look at a single particle of mass -- the smallest possible, and only that particle. Does it have gravity associated with it? Yes. Is there any force being applied? No. And there is no physical acceleration happening either. There is however a bending of space-time. So, what do we have? A gravitational field -- gravity with no force. If a mass is placed in this field both an acceleration and force simultaneously result -- even if the test mass is prevented from moving (its space-time geodesic is shifted, hence it is accelerating).

Please go here to breifly read ...
"The general theory of relativity addresses the problem of gravity and that of nonuniform, or accelerated, motion. In one of his famous thought-experiments, Einstein showed that it is not possible to distinguish between an inertial frame of reference in a gravitational field and an accelerated frame of reference. That is, an observer in a closed space capsule who found himself pressing down on his seat could not tell whether he and the capsule were at rest in a gravitational field, or whether he and the capsule were undergoing acceleration. From this principle of equivalence, Einstein moved to a geometric interpretation of gravitation. The presence of mass or concentrated energy causes a local curvature in the space-time continuum. This curvature is such that the inertial paths of bodies are no longer straight lines but some form of curved (orbital) path, and this acceleration is what is called gravitation."
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[latex]
$$
F = G \frac{m_1m_2}{r^2} =ma
$$
$$
a = G{m_2}/{r^2}
$$
$$
a = g = 9.81{m}/{s^2}
$$
[/latex]

Little help, Jimbo07?
 
Ririon said:
Whether you use the name 'gravity' for the force of gravity or the acceleration of gravity, the result should be exactly the same. If not, it is no longer a semantics debate, like "the basic equation g=F/m, clearly shows that mass is needed to produce the acceleration"...
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And from F = mg we see that mass is needed to produce a force.
 
For every quote on the net on gravity and acceleration, you will find about 12 on gravity and force. This according to the Almighty Google (which I trust more that your Britannica-quote when it comes to common word usage).

Also: Given two theoretically equivalent ways of looking at things, I choose the one that makes the most sense. You could argue that there are four basic accelerations in the universe, and that the force between two magnets is the result of the acceleration they excert on each other. But i don't like it, since they are not necessarily moving, let alone accelerating. But the force is still there.
 
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