Gravity exerts a force on everything, seemingly without running out. That last makes it seem like it could be used for a perpeetum mobile. Why, exactly, can't it?
locallyunsceneFeeder of TrollsJoin Date: 2002-12-25Member: 11528Members, Constellation
I think it's because any system that harnesses gravity must spend energy to fight against it. It's the way that you framed your question that makes it seems possible. "Particles are always attracted to each other through weak nuclear force without running out, that seems like it could be used for perpetual motion."
Another way to say it is conservation of energy plus entropy. Gravity doesn't just pull on nothing; it's an attraction between two bodies. In order to "harness gravity" you would have to be manipulating another body in a closed system with, I assume, Earth.
There are ways to non-perpetually harness gravity for energy though. Check out <a href="http://en.wikipedia.org/wiki/Tidal_power" target="_blank">tidal power</a> if you are interested.
a_civilianLikes seeing numbersJoin Date: 2003-01-08Member: 12041Members, NS1 Playtester, Playtest Lead
edited October 2009
Simple answer: The total work done by gravity around a closed loop is zero. (Gravity is a <a href="http://en.wikipedia.org/wiki/Conservative_force" target="_blank">conservative force</a>.)
A perpetual motion machine needs to end a particular cycle at the same state in which it began; otherwise it cannot continue indefinitely. Thus it may gain energy during one part of the cycle, but then it has to expend precisely that much energy to return to its starting position.
If I hold up a 1kg weight, gravity pulls on it with a force of approximately ten newton. Why can't I harness that force? Because while I hold the weight there, I am also exerting a force of approximately ten newton against gravity. Therefore, my net gain of energy is zero. The only way I can harness that force is by reducing the force I exert against gravity - in other words, letting the weight drop. This will allow me to use the kinetic energy of its downward motion to perform some amount of work. However, once the weight hits the floor, it'll come to a rest, and I can gain no further kinetic energy from it. In order to "recharge" it, I need to lift the weight back to its initial position. But in order to do that I need to exert a force greater than ten newton in order to overcome gravity. Therefore, by the time I have returned the weight to its initial position and can once more gain energy from it, all the energy I initially gained (and, inevitably, a little bit more as well) has been used on lifting the weight again.
Hydroelectric dams already use gravity to generate electricity.
The thing is, you're not generating infinite energy like this. Rather, you're spending the energy produced of the Earth's internal activity. If water falls downstream, it's because it has been brought upstream.
It IS possible to "harness" gravity, but you have to use only the "down" part of gravity. Somebody has to take care of the "bring back up" part of the cycle. At best, what YOU can do, is use the energy you've just gained to bring the pendulum back up. Factor in losses, and you're wasting energy. Pass on the job to the Earth, and you're using otherwise wasted energy.
a_civilianLikes seeing numbersJoin Date: 2003-01-08Member: 12041Members, NS1 Playtester, Playtest Lead
edited October 2009
That would cause the magnet to slow down. The current induced in the coil creates its own magnetic field which exerts a force on the magnet opposite its direction of motion (see <a href="http://en.wikipedia.org/wiki/Lenz%27s_law" target="_blank">Lenz's law</a>).
<!--quoteo(post=1730071:date=Oct 1 2009, 04:58 PM:name=Cereal_KillR)--><div class='quotetop'>QUOTE (Cereal_KillR @ Oct 1 2009, 04:58 PM) <a href="index.php?act=findpost&pid=1730071"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->[...] Pass on the job to the Earth, and you're using otherwise wasted energy.<!--QuoteEnd--></div><!--QuoteEEnd-->
Or changing the system in a possibly catastrophic way. There's no free lunch. Reminds me of that idea to put wind turbines next to a major highway and use the "wasted" airflow that traffic produces. What it would actually do is make it more difficult for traffic to maintain speed and the cars would use more fuel. In the final analysis it's more likely that the energy spent building the turbines plus the extra cost of gas would far outweigh the energy produced.
This may be going off on a tangent, but after years of initiatives to replace incandescent lightbulbs with other, less waste-heat-producing bulbs in public buildings, people have now been taken <i>completely by surprise</i> as it turns out that much of the money saved on electricity is now instead being spent on heating during the winter months.
<!--quoteo(post=1730540:date=Oct 4 2009, 12:06 AM:name=Rob)--><div class='quotetop'>QUOTE (Rob @ Oct 4 2009, 12:06 AM) <a href="index.php?act=findpost&pid=1730540"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->Or changing the system in a possibly catastrophic way. There's no free lunch. Reminds me of that idea to put wind turbines next to a major highway and use the "wasted" airflow that traffic produces. What it would actually do is make it more difficult for traffic to maintain speed and the cars would use more fuel. In the final analysis it's more likely that the energy spent building the turbines plus the extra cost of gas would far outweigh the energy produced.<!--QuoteEnd--></div><!--QuoteEEnd-->
Water turbines are pretty harmless really. The water just falls down the turbine and produce energy. The Earth brings it back up through the water cycle. They have little to no impact on anything really, considering that now they don't necessarily need huge dams that kill off ecosystems. And considering the quantity of water available, it's not the few instants added to the water flow that would change much.
When introduced into an ecosystem, water turbines need to be placed with the utmost care, lest you produce another one of those videos where a salmon futilely tries to jump against the current until it dies of exhaustion.
<!--quoteo(post=1730783:date=Oct 5 2009, 05:15 AM:name=Cereal_KillR)--><div class='quotetop'>QUOTE (Cereal_KillR @ Oct 5 2009, 05:15 AM) <a href="index.php?act=findpost&pid=1730783"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->Water turbines are pretty harmless really. The water just falls down the turbine and produce energy. The Earth brings it back up through the water cycle. They have little to no impact on anything really, considering that now they don't necessarily need huge dams that kill off ecosystems. And considering the quantity of water available, it's not the few instants added to the water flow that would change much.<!--QuoteEnd--></div><!--QuoteEEnd-->
My point, and lolf touched on it, too, is that just because energy in a system seems to serve no purpose does not mean that it's waste energy. Our conservation laws ensure that all the energy does something. Very simple things like the turbines slowing down the flow of water in the area (which they must), causing it to be more stagnant may cause a new species of bacteria to dominate the river bed or something.
I'm not saying that we should do nothing at all, that's crazy. As part of the system, us just living in it will change things. What I don't like is our poor assumption about being able to get something for nothing.
locallyunsceneFeeder of TrollsJoin Date: 2002-12-25Member: 11528Members, Constellation
<!--quoteo(post=1730800:date=Oct 5 2009, 08:19 AM:name=Rob)--><div class='quotetop'>QUOTE (Rob @ Oct 5 2009, 08:19 AM) <a href="index.php?act=findpost&pid=1730800"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->My point, and lolf touched on it, too, is that just because energy in a system seems to serve no purpose does not mean that it's waste energy. Our conservation laws ensure that all the energy does something. Very simple things like the turbines slowing down the flow of water in the area (which they must), causing it to be more stagnant may cause a new species of bacteria to dominate the river bed or something.<!--QuoteEnd--></div><!--QuoteEEnd--> Good point. One of the books I'm reading now talks about how even conservative forest management practices can be harmful to the ecosystem. There's a common practice of "culling" deadwood for logging and to prevent "pests" like bark beetles from thriving and wildfires. The problem is that this removes nutrients, shelter, and food sources from the ecosystem even though it might superficially seem beneficial.
Comments
Another way to say it is conservation of energy plus entropy. Gravity doesn't just pull on nothing; it's an attraction between two bodies. In order to "harness gravity" you would have to be manipulating another body in a closed system with, I assume, Earth.
There are ways to non-perpetually harness gravity for energy though. Check out <a href="http://en.wikipedia.org/wiki/Tidal_power" target="_blank">tidal power</a> if you are interested.
A perpetual motion machine needs to end a particular cycle at the same state in which it began; otherwise it cannot continue indefinitely. Thus it may gain energy during one part of the cycle, but then it has to expend precisely that much energy to return to its starting position.
The thing is, you're not generating infinite energy like this. Rather, you're spending the energy produced of the Earth's internal activity. If water falls downstream, it's because it has been brought upstream.
It IS possible to "harness" gravity, but you have to use only the "down" part of gravity. Somebody has to take care of the "bring back up" part of the cycle. At best, what YOU can do, is use the energy you've just gained to bring the pendulum back up. Factor in losses, and you're wasting energy. Pass on the job to the Earth, and you're using otherwise wasted energy.
Or changing the system in a possibly catastrophic way. There's no free lunch. Reminds me of that idea to put wind turbines next to a major highway and use the "wasted" airflow that traffic produces. What it would actually do is make it more difficult for traffic to maintain speed and the cars would use more fuel. In the final analysis it's more likely that the energy spent building the turbines plus the extra cost of gas would far outweigh the energy produced.
Water turbines are pretty harmless really. The water just falls down the turbine and produce energy. The Earth brings it back up through the water cycle.
They have little to no impact on anything really, considering that now they don't necessarily need huge dams that kill off ecosystems. And considering the quantity of water available, it's not the few instants added to the water flow that would change much.
They have little to no impact on anything really, considering that now they don't necessarily need huge dams that kill off ecosystems. And considering the quantity of water available, it's not the few instants added to the water flow that would change much.<!--QuoteEnd--></div><!--QuoteEEnd-->
My point, and lolf touched on it, too, is that just because energy in a system seems to serve no purpose does not mean that it's waste energy. Our conservation laws ensure that all the energy does something. Very simple things like the turbines slowing down the flow of water in the area (which they must), causing it to be more stagnant may cause a new species of bacteria to dominate the river bed or something.
I'm not saying that we should do nothing at all, that's crazy. As part of the system, us just living in it will change things. What I don't like is our poor assumption about being able to get something for nothing.
Good point. One of the books I'm reading now talks about how even conservative forest management practices can be harmful to the ecosystem. There's a common practice of "culling" deadwood for logging and to prevent "pests" like bark beetles from thriving and wildfires. The problem is that this removes nutrients, shelter, and food sources from the ecosystem even though it might superficially seem beneficial.