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Q & A: Gravity as an energy source?

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Most recent answer: 04/10/2019
Q:
Just to know, would it be possible to use gravity as a power source using gravity and magnetism or gravity and pietzo-electricity to produce energy? in theory i think this should work to creat a new sustainable energy, no? would it be too expensive for a weak power? why, in my knowledge, nobody used that?
- Eric (age 21)
France
A:
Dear Eric,

First of all, thank you very much for your question! You are certainly doing some good thinking.

While perhaps intuitively this idea might sound plausible, the answer to your question is no.

While gravitational energy can be converted into other forms of energy, the only way you can get some energy out of a gravitational field is to move from an area of high gravitational potential to an area of low gravitational potential (in other words, moving from a high place to a low place). For example--when you drop a ball, its change in gravitational potential energy gets converted into kinetic energy until it hits the ground. You have to use the exact same amount of energy you got from the fall to get the ball back to the height it started so it can fall again. You don't gain any net energy.

Is there any weasel room here? With clever thinking can we maybe come up with some contraption to work around this? No.

The reason being that this sort of thing is exactly the same (in principle) as and clearly violates fundamental thermodynamical laws. It's important to take into account the TOTAL energy gained or lost in any system.

For instance, using the sun as an energy source works because we have a consistent influx of electromagnetic energy due to the nuclear fusion reactions which power the sun.

I hope that answers your question!

Sincerely,
John

(published on 02/03/2011)

Follow-Up #1: gravity and the sun's nuclear fusion

Q:
But the fusion in the Sun's core is due to the gravitationnal collapse of the Sun (with the help of quantum tunnelling). In that sense, gravity isn't an energy source but it does play an important role. No gravity, no Sun, Earth or anything. Only a dull, freezing, dead Universe constantly expanding...
- Anonymous
A:
Ok, fair enough.

Mike W.

(published on 02/17/2011)

Follow-Up #2: magnets, gravity, and energy conservation

Q:
Hi there, I see that this post is several years old now, so I don't know if it's still active, but I did have a follow up question to the original question & answer..As stated in the answer to the original question, "Is there any weasel room here? With clever thinking can we maybe come up with some contraption to work around this? No.The reason being that this sort of thing is exactly the same (in principle) as and clearly violates fundamental thermodynamical laws. It's important to take into account the TOTAL energy gained or lost in any system."Perhaps I don't understand this explanation, but here's my follow-up question anyway..All I understand is that when objects fall to the earth (such as water from a waterfall) & gain enough momentum with the help of gravitational forces, energy is able to be "harvested" from them & then converted to electrical stuffs. In addition to this, I also understand that magnetized objects have an ability to push certain other objects away from them, even against the larger gravitational force of this lovely planet. Why couldn't we build some series of magnets (like a stairway) that pushes some free -moving magnetized object from a lower point to a higher point & then let that free moving object fall freely in order to "harvest" the energy from its falling motion? ...and then start the whole climbing-falling process over again..If someone is able to respond in fairly simple terms that would be helpful - thanks. Also, a response of "it violates the basic laws of thermodynamics" doesn't mean anything to me. But why would this violate thermodynamic laws as we know them?
- Matthew Romans (age 27)
U.S.
A:

The magnets would have to supply energy to push the thing back up. You could describe how the potential energy varies as a function of position by some function U(r). Things accelerate toward wherever U is smallest, and generally lose a little energy to friction on the way. That makes them gradually actually settle down to the lowest-U point. You know that happens when U comes from just gravity.

Adding some magnets may complicate the form of U a bit, but it doesn't change that qualitative behavior.  For magnets U may be a function not only of position r but also of the orientation Θ of the object. So the onbject falls and turns to end up at the lowest U(r,Θ). It's still the same idea.

Mike W.


(published on 10/04/2016)

Follow-Up #3: energy from gravity

Q:
If I'm not mistaken, I believe gravity was thought to be the source of energy for the sun for a very significant amount of time. In the 1890's Lord Kelvin originally estimated the age of the sun to be 100 million years old and then corrected his calculations to conclude that if the sun was powered by gravity, it would be about 30 million years old. He calculated the sun's energy using the equation (GM^2)/R which is the equation for energy contained in a radius. This was later corrected because people discovered that the sun was in fact, powered by Fusion which was much more efficient than Gravity and Fission. So is gravity not a viable energy source?
- Jackson Douglas (age 15)
New York, NY, USA
A:

You're absolutely right. A cloud of masses does lower its gravitational potential energy as it collapses to a lump. Historically, that seemed like a possible source of the energy radiated by the Sun, until it was realized that the time scale wasn't close to what was needed- in other words the total energy releaased was too small to have kept the Earth warm for as long as the fossil/geological record says it has been. So yes, like other position-dependent types of potentail enrgy, gravity can supply energy output until things have collapsed to their low-energy positions. 

In practice, the Sun's nuclear fuel has far more energy to release before it's used up than do any nearby gravitational sources.

Mike W.


(published on 04/10/2019)

Follow-up on this answer.