Energy from Gravity?

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Nebuduck
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Energy from Gravity?

Postby Nebuduck » Mon Apr 14, 2008 10:38 pm UTC

So, I'm not entirely sure where or if my logic is going wrong here, but it doesn't seem quite right...

Let us assume we have a tube full of a liquid or gas of density P, a solid ball of material of density Q, and another tube of a liquid of gas of density R. These are all on Earth's surface (or some other body with mass, it doesn't really matter).

Now, let P<Q<R

Evidently, if we put the ball in the first tube of the P material, the ball will fall to the bottom. If we post the ball into the bottom of the other tube, it'll rise to the top. This motion will generate some degree of kinetic energy, and also some energy will be lost as work done against resistances. The work done energy will increase indefinitely with the length of the tube, producing heat and suchlike, which could be "harvested", while the kinetic energy will reach a maximum (terminal velocity).

Now, lets put these tubes next to each other, and have some device to move the ball from tube of density P to tube of density R at the bottom, and back again at the top. It may not be possible to make it perfect with current technology, since it should move the ball, and none of the surrounding material, but it is possible to conceive of such a device. We can assume that the device requires a constant amount of energy to operate.

So, surely, we can make each tube as long as we like, increasing the amount of energy produced in kinetic and work done indefinitely. But the device at the bottom and top will use a certain amount of energy however long the tube may be, so it is definitely possible to increase the tube to a length such that the energy requirements of the device are less than the energy released.

Now, I can see that this is evidently getting energy from an outside source - gravity. But nonetheless, it seems to be obtaining this energy without reducing the amount of gravity, so it can draw energy indefinitely - unlike, for example, fusion in a star, which will eventually burn itself out.

So, this seems to be a little bit odd. Is there a problem in my logic, or is this a valid method of long term energy production?

(Note: a similar thing could be carried out with any of the other forces, not just gravity - you'd just need materials which were affected by the force a different amount - maybe ionic solutions of different charges would work in an electromagnetic field, though I'm not entirely sure about that.)

Edit: changed "fission in a star" to "fusion in a star". I really must try to remember my basic astrophysics...
Last edited by Nebuduck on Mon Apr 14, 2008 11:02 pm UTC, edited 1 time in total.

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Re: Energy from Gravity?

Postby Benitosimies » Mon Apr 14, 2008 10:51 pm UTC

You should probably stick with liquid for the P tube. In my experience it's harder to float things with gases than with liquids.

Little thought: If you line the ball and tube with the right things you could generate static electricity (I assume).

EDIT: What's the plan for putting the ball at the bottom of the tube full of liquid without the liquid spilling out?
Or, if it's a lighter-than-air gas, without the gas spilling out?
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Re: Energy from Gravity?

Postby Nebuduck » Mon Apr 14, 2008 10:57 pm UTC

Well, it's more a thought experiment than anything I ever plan to actually try, seeing as how I can't stick two sheets of paper together without causing the universe to implode. But the static sounds like a good idea, if possible. I'd always imagined some sort of system of little rotors that the ball turned as energy gathering mechanisms, but there should be all kinds of ways of doing it. To be honest, it's an idea that I've had since I was about 7, and I've never been able to reconcile with my ideas about conservation of energy, so I thought I'd just fling it out there.

Edit in response to your Edit: Well, some kind of valve sort mechanism on the bottom - that's why I said the device would be plausible, but not necessarily easy to make. I kind of imagined it as one long elongated-doughnut tube with some sort of mechanised valve at the bottom to post the ball through, but it seemed easier to explain in this way.

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Re: Energy from Gravity?

Postby Micron » Mon Apr 14, 2008 11:00 pm UTC

Buoyancy problems are a classic perpetual motion blunder and almost all of them are based on the same flaw. You are failing to account for all of the work done as the ball sinks or rises. As the ball sinks down your tube what happens to the gas or fluid around it? The ball displaces a certain volume of fluid, as the ball descends an equal volume of fluid must be pushed upwards to fill the space the ball occupied a moment ago. Reverse the directions of the opposite case of the ball rising.

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Re: Energy from Gravity?

Postby Lycur » Mon Apr 14, 2008 11:03 pm UTC

My gut feeling is that the problem with this is keeping your fluids separated. The greater the height of your tube the greater the force acting to bring them to equilibrium. You need to put in increasing energy to oppose that. But, as I say, that's just my gut feeling and I wouldn't stand too passionately by it.

As for energy from gravity, I can't for the life of me remember/figure out how exactly it works. As a less abstract example, two bodies orbiting each other will produce tidal waves. These waves clearly have kinetic energy. Where does it come from?

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Re: Energy from Gravity?

Postby skeptical scientist » Mon Apr 14, 2008 11:04 pm UTC

When you place the ball in the bottom of the second tube, you are displacing some amount of water, which raises it an amount h. The important thing is that it raises every bit of water in the tube the same amount, so the total energy needed depends on the height of the tube. So no matter how high the tube is, you never extract any energy, and in fact lose a bit of energy to heat from friction.
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Re: Energy from Gravity?

Postby Nebuduck » Mon Apr 14, 2008 11:07 pm UTC

Micron wrote:Buoyancy problems are a classic perpetual motion blunder and almost all of them are based on the same flaw. You are failing to account for all of the work done as the ball sinks or rises. As the ball sinks down your tube what happens to the gas or fluid around it? The ball displaces a certain volume of fluid, as the ball descends an equal volume of fluid must be pushed upwards to fill the space the ball occupied a moment ago. Reverse the directions of the opposite case of the ball rising.

(and skeptical scientist and everyone with similar)

Ah, thanks. I see what you're saying - makes sense. I've been wondering for ages where the flaw in that logic was, it's always something relatively obvious...

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Re: Energy from Gravity?

Postby Micron » Mon Apr 14, 2008 11:15 pm UTC

The Museum of Unworkable Devices has a whole collection of similar ideas and puzzles that might amuse you. It looks like they even have a variant of your suggestion.

It's amazing how clever people can be at complicating the problem without ever addressing the fundamental flaws.

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Re: Energy from Gravity?

Postby ducksan » Mon Apr 14, 2008 11:48 pm UTC

Well, a perpetual motion device is possible - from the perspective of the Earth system, continuously receiving solar energy from the Sun's nuclear fusion. Hopefully we'll be able to make these devices for our alternative fuels in the future, when petroleum energy is a thing of the past.
But yeah, the alchemists shit themselves over trying to find a way to make a machine run forever, as with turning lead into gold, et cetera. At least they might've sorta paved the way for modern chemistry.
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Re: Energy from Gravity?

Postby BlackSails » Tue Apr 15, 2008 12:26 am UTC

ducksan wrote:Well, a perpetual motion device is possible - from the perspective of the Earth system, continuously receiving solar energy from the Sun's nuclear fusion.


The sun's fusion isnt perpetual.

Why exactly would dieletric wheel at http://www.lhup.edu/~dsimanek/museum/advanced.htm not work?

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Re: Energy from Gravity?

Postby skeptical scientist » Tue Apr 15, 2008 1:58 am UTC

BlackSails wrote:Why exactly would dieletric wheel at http://www.lhup.edu/~dsimanek/museum/advanced.htm not work?

The forces aren't as claimed. The first equation is valid if there is no dialectric present, and the second is valid if dialectric permeates all of space. Since neither are the case, you have to have a more complicated formula, which is essentially the same in the first case, but must be radically modified in the second case for objects near the surface of the water, for which the force is much greater and in a different direction.

Once you do this, the force field becomes conservative, you can introduce a potential, and the wheel will settle into a minimum (or locally minimal) potential state and thenceforward feel no torque. Physics works; all is well.

The one that really got me was the energy sucker. I had to look up the answer for that one.

By the way, elsewhere in the museum there is an illustration of your device:
The Museum of Unworkable Devices wrote:Image
The hydraulic ball-machine. This artist's conception represents an idea patented several times. Balls lighter than water rise in the tank at the right, spilling over and then by their own weight drive a vertical conveyor belt at the left. Some "Rube-Goldberg" linkages control several levers that operate "magic" valves in the water column to control the rate of transport of the balls. And even more mysterious "magic" valve allows the balls to pass from the bottom of the conveyor belt into the water tank. [Science and Invention, March 1925.]
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Re: Energy from Gravity?

Postby evilbeanfiend » Tue Apr 15, 2008 8:56 am UTC

of course if you really do want to get energy from gravity -> hydro-electric dam! the sun kindly generates the energy necessary to keep you supplied with water at the top of the dam.
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Re: Energy from Gravity?

Postby Nebuduck » Tue Apr 15, 2008 4:00 pm UTC

Of course, the majority of the energy we use can be traced back to gravity at some point.

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Re: Energy from Gravity?

Postby Badion » Tue Apr 15, 2008 4:24 pm UTC

Very interesting, and some GREAT! responses. I believe Mythbusters tackled this on an episode. They did a great job explaining how displacement really effects the amount of work being done.
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Re: Energy from Gravity?

Postby ducksan » Sun Apr 27, 2008 5:52 pm UTC

BlackSails wrote:The sun's fusion isnt perpetual.


By the time it stops, we'll be in enough trouble that we won't notice and/or care. Or exist. >.>
I think billions of years is close enough to perpetual, for our purposes.
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Re: Energy from Gravity?

Postby BlackSails » Sun Apr 27, 2008 11:13 pm UTC

ducksan wrote:
BlackSails wrote:The sun's fusion isnt perpetual.


By the time it stops, we'll be in enough trouble that we won't notice and/or care. Or exist. >.>
I think billions of years is close enough to perpetual, for our purposes.


Perpetual means forever, not a very long time.

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Re: Energy from Gravity?

Postby zenten » Mon Apr 28, 2008 1:51 pm UTC

Lycur wrote:As for energy from gravity, I can't for the life of me remember/figure out how exactly it works. As a less abstract example, two bodies orbiting each other will produce tidal waves. These waves clearly have kinetic energy. Where does it come from?


It slows down the rotation. Actually, what happens is the two bodies eventually become tidally locked (barring them being in a very long term unsteady orbit or something). That's why the moon shows pretty much the same face all the time, with a bit of wiggle. There used to be a lot more wiggle once, and later from now the wiggle will be less.

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Re: Energy from Gravity?

Postby aesxd » Mon Apr 28, 2008 4:27 pm UTC

Having that configuration in the first place is difficult. What is stopping the denser liquid from flowing from the right tube into the left tube? If they are separated, then the ball will not be able to move between the two tubes. If there is an opening at the bottom of either tube, the liquid would flow out of it. Perhaps some sort of valve would work..

If, somehow, you were able to even assemble this, I think that the energy that would have been extracted would have to be expended to move the ball from the lighter fluid into the denser one. The ball would just "float" on the boundary between the two fluids if it wasn't being forced in.

Of course, correct me if I'm wrong about any of my assumptions

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Re: Energy from Gravity?

Postby Xanthir » Mon Apr 28, 2008 6:50 pm UTC

aesxd wrote:Having that configuration in the first place is difficult. What is stopping the denser liquid from flowing from the right tube into the left tube? If they are separated, then the ball will not be able to move between the two tubes. If there is an opening at the bottom of either tube, the liquid would flow out of it. Perhaps some sort of valve would work..

If, somehow, you were able to even assemble this, I think that the energy that would have been extracted would have to be expended to move the ball from the lighter fluid into the denser one. The ball would just "float" on the boundary between the two fluids if it wasn't being forced in.

Of course, correct me if I'm wrong about any of my assumptions

That's exactly why this machine doesn't work. The amount of energy required to force the ball under the lip and into the water-column is at least as much as is produced by the ball falling.
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Re: Energy from Gravity?

Postby ducksan » Fri May 02, 2008 1:56 am UTC

BlackSails wrote:
ducksan wrote:
BlackSails wrote:The sun's fusion isnt perpetual.


By the time it stops, we'll be in enough trouble that we won't notice and/or care. Or exist. >.>
I think billions of years is close enough to perpetual, for our purposes.


Perpetual means forever, not a very long time.


I hate to beat a dead horse, but how much does proving me wrong/assuming I don't know how to define "perpetual" MEAN to you? If it's after humans die out, we shouldn't even worry about it except for scientific curiosity. As long as humans exist, we will have solar energy, unless something extremely bizarre happens. This is what I've been trying to say.

Actual thread-related comments: I can't think of many. Gravity certainly ain't no source of free energy, unless it's solar-provided like I said earlier. A lot of hydroelectric stuff is simply gravity; I can't think of a much better way to get gravitational energy. You could push a rock up a cliff and drop it onto something that would harness its KE. I'm not very good at imagining these kinds of things, and I hope someone will be able to find more ways to harness the release in energy when something moves closer to Earth's core.
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Re: Energy from Gravity?

Postby antonfire » Fri May 02, 2008 2:20 am UTC

zenten wrote:
Lycur wrote:As for energy from gravity, I can't for the life of me remember/figure out how exactly it works. As a less abstract example, two bodies orbiting each other will produce tidal waves. These waves clearly have kinetic energy. Where does it come from?


It slows down the rotation. Actually, what happens is the two bodies eventually become tidally locked (barring them being in a very long term unsteady orbit or something). That's why the moon shows pretty much the same face all the time, with a bit of wiggle. There used to be a lot more wiggle once, and later from now the wiggle will be less.
Oh, man, I wish I could have been there for the first time that it didn't quite spin around, and wiggled back instead. That would be such a "holy crap" moment.
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Re: Energy from Gravity?

Postby EricH » Fri May 02, 2008 2:13 pm UTC

antonfire wrote:
zenten wrote:
Lycur wrote:As for energy from gravity, I can't for the life of me remember/figure out how exactly it works. As a less abstract example, two bodies orbiting each other will produce tidal waves. These waves clearly have kinetic energy. Where does it come from?


It slows down the rotation. Actually, what happens is the two bodies eventually become tidally locked (barring them being in a very long term unsteady orbit or something). That's why the moon shows pretty much the same face all the time, with a bit of wiggle. There used to be a lot more wiggle once, and later from now the wiggle will be less.
Oh, man, I wish I could have been there for the first time that it didn't quite spin around, and wiggled back instead. That would be such a "holy crap" moment.


Not that dramatic, I'm afraid--more like, spinning slower and slower. The wiggle we see now would have appeared as an irregularity in its (very slow) rotation, that it turned slightly faster or slower during its cycle. Eventually, the speed would be so low that the slowdown brought it all the way to zero, but to an observer on any human time scale, the difference between 'barely turning' and 'not turning' would be almost imperceptible.
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Re: Energy from Gravity?

Postby antonfire » Fri May 02, 2008 5:24 pm UTC

Hm, I had assumed that it was something like a pendulum spinning about its axis. It would keep making complete revolutions about its axis and losing energy, until at some point, it didn't have quite enough, and failed to make a complete spin.

But yes, I can see how that might be wrong.
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Re: Energy from Gravity?

Postby andyhenault » Sat May 03, 2008 3:33 am UTC

Alright, this has sort of been addressed, but heres the exact answer. When the ball is placed at the bottom of the column of water, it is elevating the entire column of water above it. It takes a certain amount of energy to place this ball at the bottom of the tube since the water now has a higher gravitational potential energy. This is the critical error in the apparatus, as it is assumed that the ball will slip in effortlessly.

One person suggested that a series of paddles linked to a vertical conveyer could be used to harness the energy of the ball rising. The total energy gained by these means is equal to FxD,F being the buoyancy force of the ball, and D being the height of the tube. Since the maximum possible energy from buoyancy is obtained when the weight of the ball is equal to 0, the buoyant force of the ball in the best case scenario is mgV (mass*gravity*volume). Now, the energy derived from the rising ball is E=mgVD. As discussed earlier, the change in potential energy of the column is equal to the total weight of the column of water multiplied by the increase in height, or, the increase in weight multiplied by total height, this is also equal to mgVD.

Hence, the energy required to elevate the column of water is exactly equal to the amount of energy derived from the rising ball, in the best case scenario. But by all means, build your machine and defy hundreds of years of thermodynamics.
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Re: Energy from Gravity?

Postby simdude » Sat May 03, 2008 7:49 pm UTC

I'd been doing some research for an english paper about scientific scams so I think I've had my fill of perpetual motion devices, but the devices do make me think "What is the best possible way to extract clean efficient energy?" And I honestly wish that's what more time was spent thinking about instead of trying to beat down the laws of thermodynamics.

I've been toying with a thought experiment. Immagine you have a giant rod with a ball on the end. That ball is able to slide somewhat freely (but would need to be able to be controlled). The idea would be the rod begins turning and the ball is slowly moved towards the center of the rod as it spins increasing it's kinetic energy and hence output. When the ball reaches the center manual labor could be used to push it back to its original starting point. So by using manual labor you're not breaking any laws, but what I'm curious about is how hard it would be to control the ball's movement. I don't know if the ball would naturally want to accelerate inwards or outwards and if the force required to keep it moving slowly and stably inwards would be greater than the output of the machine in the end.

Anyone care to help elaborate on that for me?

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Re: Energy from Gravity?

Postby andyhenault » Sat May 03, 2008 9:52 pm UTC

simdude wrote:I'd been doing some research for an english paper about scientific scams so I think I've had my fill of perpetual motion devices, but the devices do make me think "What is the best possible way to extract clean efficient energy?" And I honestly wish that's what more time was spent thinking about instead of trying to beat down the laws of thermodynamics.

I've been toying with a thought experiment. Immagine you have a giant rod with a ball on the end. That ball is able to slide somewhat freely (but would need to be able to be controlled). The idea would be the rod begins turning and the ball is slowly moved towards the center of the rod as it spins increasing it's kinetic energy and hence output. When the ball reaches the center manual labor could be used to push it back to its original starting point. So by using manual labor you're not breaking any laws, but what I'm curious about is how hard it would be to control the ball's movement. I don't know if the ball would naturally want to accelerate inwards or outwards and if the force required to keep it moving slowly and stably inwards would be greater than the output of the machine in the end.

Anyone care to help elaborate on that for me?


I'm not really sure what type of device your talking about, but I think it relates to a simple conservation of angular momentum problem. Exactly how is this bar spinning, and where is the ball?
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Re: Energy from Gravity?

Postby simdude » Sat May 03, 2008 11:12 pm UTC

andyhenault wrote:
simdude wrote:I'd been doing some research for an english paper about scientific scams so I think I've had my fill of perpetual motion devices, but the devices do make me think "What is the best possible way to extract clean efficient energy?" And I honestly wish that's what more time was spent thinking about instead of trying to beat down the laws of thermodynamics.

I've been toying with a thought experiment. Immagine you have a giant rod with a ball on the end. That ball is able to slide somewhat freely (but would need to be able to be controlled). The idea would be the rod begins turning and the ball is slowly moved towards the center of the rod as it spins increasing it's kinetic energy and hence output. When the ball reaches the center manual labor could be used to push it back to its original starting point. So by using manual labor you're not breaking any laws, but what I'm curious about is how hard it would be to control the ball's movement. I don't know if the ball would naturally want to accelerate inwards or outwards and if the force required to keep it moving slowly and stably inwards would be greater than the output of the machine in the end.

Anyone care to help elaborate on that for me?


I'm not really sure what type of device your talking about, but I think it relates to a simple conservation of angular momentum problem. Exactly how is this bar spinning, and where is the ball?


Well I can't really think of a device which opperates like this, but I'll try to spell my thought experiment out more. First. Yeah the whole idea behind this would be that angular momentum is going to be conserved and so it will continue to speed up putting out more and more kinetic energy, which I'm guessing could be quite large. I'll do the calculations in a little. I should have mentioned similar to how the ball is reset manually the bar would be starts spinning with manual labor as well. The ball would be positioned on the end and would somehow (through either a combination of either natural or mechanical forces, which I am curious how possible it is) slowly be moving towards the axis of rotation as it spins.

While that was a fun thought experiment, I just did some calculations and I hadn't taken into account just how impossible it would for manual labor to get a decent sized machine started.

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Re: Energy from Gravity?

Postby btarlinian » Sat May 03, 2008 11:24 pm UTC

simdude wrote:I'd been doing some research for an english paper about scientific scams so I think I've had my fill of perpetual motion devices, but the devices
I've been toying with a thought experiment. Immagine you have a giant rod with a ball on the end. That ball is able to slide somewhat freely (but would need to be able to be controlled). The idea would be the rod begins turning and the ball is slowly moved towards the center of the rod as it spins increasing it's kinetic energy and hence output. When the ball reaches the center manual labor could be used to push it back to its original starting point. So by using manual labor you're not breaking any laws, but what I'm curious about is how hard it would be to control the ball's movement. I don't know if the ball would naturally want to accelerate inwards or outwards and if the force required to keep it moving slowly and stably inwards would be greater than the output of the machine in the end.

Anyone care to help elaborate on that for me?


Well the ball wouldn't naturally move inwards. In fact, without any force holding it onto the rod, it would fly off the end of the rod. You can try this yourself. Hold something heavy in both your hands and hold them away from your body while spinning in an office chair. Now pull them in. You'll notice that takes more effort than pulling your hands in when you are not spinning. Essentially, something has to provide a centripetal force to push the weight towards the center. If the ball is affixed to the rod, then the tension within the rod will do this. If it can move, something else would need to exert a constant force to hold this in. And this force would increase as you got closer weight moved closer towards the middle since [imath]a_c=r\omega^2[/imath] and as you push something in, the radius and angular velocity are inversely proportional to one another.

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Re: Energy from Gravity?

Postby whitedevilbrewing » Sun May 04, 2008 3:55 am UTC

Reminds me of this- http://www.core77.com/competitions/greenergadgets/projects/4306/

It's easy to forget a few basic principles and think you have the big answer. It's good homework as long as you don't become so deluded that you actually enter it into a design competition and... win? Ya, this thing won despite the fact that there's no way it could work as claimed, which reflects worse on the competition than on the designer perhaps.

http://www.vtnews.vt.edu/story.php?relyear=2008&itemno=111 Criticized there.

It has a lot in common with perpetual motion machines, where the linchpin is always just around the corner (or more insidiously, requiring just a bit more money from gullible investors...)

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Re: Energy from Gravity?

Postby SabreKGB » Sun May 04, 2008 5:18 am UTC

Nebuduck wrote:Of course, the majority of the energy we use can be traced back to gravity the sun at some point.

Fix'd

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Re: Energy from Gravity?

Postby SabreKGB » Sun May 04, 2008 5:24 am UTC

whitedevilbrewing wrote:Reminds me of this- http://www.core77.com/competitions/greenergadgets/projects/4306/

It's easy to forget a few basic principles and think you have the big answer. It's good homework as long as you don't become so deluded that you actually enter it into a design competition and... win? Ya, this thing won despite the fact that there's no way it could work as claimed, which reflects worse on the competition than on the designer perhaps.


Ok, i have to ask this sheepishly, but why wouldn't this particular device work? Is it a flaw in concept, or engineering? I looked at it and nothing jumped out at me.

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Re: Energy from Gravity?

Postby Antimony-120 » Sun May 04, 2008 5:44 am UTC

Oddly I have the Gravia thing as coming in Second on the competition, and that's what's being criticized in your review.

The main problem is that they've ignored the fact that the weight can't provide that much power output for that long when suspended to that height.

Equivalent to a 40 watt bulb = 40J/s

For 4 hours
40J/s * (4*60*60) = 576000J

48" = 4' = approx 1.22m

W = mgh (at eff = 1) = m*1.22*9.81 = 576000J

m = 48127.54kg = 24.06 metric TONS.

Users will have to be quite strong to lift that weight.

(Note: somebody check my math, I worked 10 hrs today so I might be crazy)
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Re: Energy from Gravity?

Postby phlip » Sun May 04, 2008 5:57 am UTC

SabreKGB wrote:
whitedevilbrewing wrote:Reminds me of this- http://www.core77.com/competitions/greenergadgets/projects/4306/

It's easy to forget a few basic principles and think you have the big answer. It's good homework as long as you don't become so deluded that you actually enter it into a design competition and... win? Ya, this thing won despite the fact that there's no way it could work as claimed, which reflects worse on the competition than on the designer perhaps.


Ok, i have to ask this sheepishly, but why wouldn't this particular device work? Is it a flaw in concept, or engineering? I looked at it and nothing jumped out at me.


Well, there isn't much information on that page, that I could see, but going by what is there... a weight falling 48", powering a 600-800lm LED for 4 hours.

Now, let's go with the extremes... go with 600lm, and according to Wikipedia, some experimental LEDs can get as high as 150lm/W, so this LED will draw 4W, at a minimum. Over 4 hours, that'll be 4*4*60*60 = 57600J.

So, an object falling 1.2m has to provide 57.6kJ... 57600 = mgh, m = 57600/(1.2 x 9.8) = just under 5 tonnes.

That is, assuming a minimum amount of power needed to drive the light, and a perfect conversion of the gravitational potential into electric energy, you'd still need a 5t mass to do the things the device is claiming. To get up to 800lm, or if you're using a less efficient LED, or there are any losses elsewhere in the system, then the weight'll have to be even heavier.

[edit]
I see from here that the weights are only 50lb... so definitely not going to happen. 50lb could light those LEDs for about 2.5 minutes.

Also, ninjaed by Antimony, but note that the LEDs don't draw 40W... the "40W" figure is what an incandescent bulb would draw for the same light output. I came up with 4W, so we're out by a factor of 10. (Also, a tonne is 1000kg, not 2000kg.)

[edit again]
From the text at the bottom of that image I linked to, it looks like the designer thinks that spinning the generator faster == more power (see the bits about using gears to make it spin faster). Unfortunately spinning the generator faster also means needing more force to make the generator turn, and gearing up the speed reduces the amount of force you can get from your falling weight (in any machine, force ratio * speed ratio ≤ 1, with equality only in a perfect world). This reduction in the force you can get from your falling weight can be counteracted by making your falling weight heavier (say, 5 tonnes).

[edit yet again]
From that vtnews link that whitedevilbrewing linked to, it says that it's currently only theoretical, until low-power light sources get better. However, again citing that same Wikipedia page, the theoretical maximum lm/W value is 683 lm/W (which happens when 100% of the incoming power is converted to green light at 555nm - the peak of the average human eye's response curve). Plugging that into the calculations above still gives just over a tonne as necessary to power the system.

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Re: Energy from Gravity?

Postby whitedevilbrewing » Sun May 04, 2008 6:45 am UTC

Ya, you guys nailed it. And it came in second? My poor recollection then :P Anyway, it gives you an idea of the validity of that particular competition!

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Re: Energy from Gravity?

Postby SabreKGB » Sun May 04, 2008 8:03 am UTC

Ok, i thought that the objection would be engineering in nature. The concept is sound, the device is just not practical. I didn't see any numbers on the page i was looking (glanced) at either, so i didn't really have anything to go on.

It boils down to: Dude fucked up (or didn't do) his math.

*Thanks for the lesson. I need to pick up more engineering, so i can do these sort of analyses myself*

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Re: Energy from Gravity?

Postby Steax » Sun May 04, 2008 9:03 am UTC

I made this quick machine up to have some fun at school tomorrow. Where do you guys think the flaw here will be in? I think it'll be in calibrating the magnets, so that it enters the heavy liquid with enough force, but is then enough to get it to float back up. I suppose electromagnets are the only solution here, then. :mrgreen:
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Re: Energy from Gravity?

Postby phlip » Sun May 04, 2008 9:06 am UTC

Well, for a start, all your fluid's gonna flow out of the hole at the bottom where the ball goes in... water pressure, and all that...

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Re: Energy from Gravity?

Postby Steax » Sun May 04, 2008 9:12 am UTC

Well ok, raise the right side of the heavier fluid (sorry, missed that one, lol). The heavier weight of that fluid should be enough to stabilize the weight of the lighter one.
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Re: Energy from Gravity?

Postby Micron » Sun May 04, 2008 6:04 pm UTC

Your ball still has to lift its own volume worth of liquid the full height of the tube when it enters the liquid. The ball floating up the tube is equivalent to this volume of liquid falling back down.
Steax wrote:Well ok, raise the right side of the heavier fluid (sorry, missed that one, lol). The heavier weight of that fluid should be enough to stabilize the weight of the lighter one.

The ball will be more buoyant in a heavier fluid, it will take more work to submerge it and move it toward the tube full of liquid.

Really for this sort of setup you might as well just describe a bouncing ball. Under impossibly perfect conditions you could drop a ball and have it always bounce back to its original height. Adding liquids and ramps and such just obscures where the energy changes back and forth from potential to kinetic. Have fun confusing your classmates though.


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