Question

**Lemur** · 06-09-2006, 19:26

Perpetual motion should be filed next to the Philosopher's Stone, just across from the Elixir of Life. If you reach cold fusion you've gone too far, and should turn back.

**Uesugi Kenshin** · 06-09-2006, 22:14

So long as there is something other than the perpetual motion device it will not be possible.

**Kralizec** · 06-09-2006, 22:18

Originally Posted by Lemur

Perpetual motion should be filed next to the Philosopher's Stone, just across from the Elixir of Life. If you reach cold fusion you've gone too far, and should turn back.

If I remember right, there's one (theoretical) instance of perpetual motion: when one celestial object circles around the other, ignoring the gravitational pul from other objects. Since there's no friction of atmosphere, it could move on forever- it's impossible to make it into an energy source though, because that would slow down the orbit.

**Avicenna** · 06-10-2006, 07:10

It's impossible to ignore gravity though, and the movement will inevitably cause heat, causing it to lose heat and therefore eventually run out of energy.

Perpetual motion is impossible.

**Duke of Gloucester** · 06-10-2006, 07:50

Originally Posted by Tiberius

It's impossible to ignore gravity though, and the movement will inevitably cause heat, causing it to lose heat and therefore eventually run out of energy.

Perpetual motion is impossible.

Please describe the mechanism by which gravity generates heat.

Perpetual motion is possible in theory, but you can't harvest any energy from the system because this would slow it down.

**macsen rufus** · 06-10-2006, 17:10

There's three laws of thermodynamics that need to be broken before perpetual motion has a chance.

1 - you can' t win
2 - you can't break even
3 - you can't get out of the game

By definition it'll never be an energy source, soon as you extract any energy it ceases to be perpetual (sheesh, why is perpetual so difficult to type??!)

If you reach cold fusion you've gone too far, and should turn back.

What?? You mean all that stuff I'm building in the attic won't work after all, damn you Lemur, and your bad news

**Avicenna** · 06-11-2006, 08:36

Originally Posted by Duke of Gloucester

Please describe the mechanism by which gravity generates heat.

Perpetual motion is possible in theory, but you can't harvest any energy from the system because this would slow it down.

Like macsen rufus said, if you make a perpetual motion machine it wouldn't be an energy source. Also, Gravitational Potential Energy converting to kinetic is an energy loss, is it not? There are also inevitably some particle scattered here and there, and friction between the object and particle converts, and hence loses energy.

GC: interesting idea, I'll look into it. Even if it is though, how would we extract energy? It will also not be for long, because eventually the Sun is going to engulf the Earth I think. Even if it doesn't and stops expanding at Mars, it's going to be far too hot for us to live here.

**Duke of Gloucester** · 06-11-2006, 09:02

Originally Posted by Tiberius

Like macsen rufus said, if you make a perpetual motion machine it wouldn't be an energy source.

Yes. I said this too.

Originally Posted by me

Perpetual motion is possible in theory, but you can't harvest any energy from the system because this would slow it down.

Originally Posted by Tiberius

Also, Gravitational Potential Energy converting to kinetic is an energy loss, is it not?

Definitely not. It is a conversion from GPE to kinetic which will in an eliptical orbit will be converted back to GPE again. However KE or GPE degraded to heat, although not lost, cannot be wholly converted back into KE or GPE (2. You cannot even break even.)

There are also inevitably some particle scattered here and there, and friction between the object and particle converts, and hence loses energy.

This is true.

What is the mechanism for this?

Originally Posted by Tiberius

GC: interesting idea, I'll look into it. Even if it is though, how would we extract energy? It will also not be for long, because eventually the Sun is going to engulf the Earth I think. Even if it doesn't and stops expanding at Mars, it's going to be far too hot for us to live here.

Exactly the same thing that Mascen Rufus and I said about perpetual motion applies to the Earth/Sun system. In fact tidal power does extract energy from the earth's rotation and its linear kinetic energy as it moves around the sun. The earth's KE/PE is degraded and warms the oceans. It is far more signifcant way of the earth losing it's orbital/rotational energy than collisions with particles. This will continue until the Earth's period of rotation matches its orbital period. Then the Earth will lose orbital energy in the way you described. Before the energy is completely lost, the Sun will pass in to its red giant stage.

**doc_bean** · 06-11-2006, 09:33

Originally Posted by Gelatinous Cube

That's pretty interesting. Alright, forgive my physics noob-ism once more:

Would it not be theoretically possible to fashion some device that harnesses that same force which causes tidal changes and whatnot into a conventional energy you could use to power computers and heat homes?

Gravity ? There have been some experiments to use the energy stored in gravitational fields, but afaik none were a real success. I remember seeing a report once about some guy claiming he had build an engine that worked by getting energy from the motion of the stars

Gravity is very important for energy storage though, the way we do it here, and I imagine in quite a few other places, is to pump water from a lower to a higher tank, and then later use the energy from letting the water come back down again.

Tidal wave energy generation exist, I believe they use it in some parts of Britain.

BTW Does anyone know at what speed gravity 'travels' ? Is it instantaneous or is it, like light, limited in speed ?

**Banquo's Ghost** · 06-11-2006, 11:26

Originally Posted by Gelatinous Cube

Would it not be theoretically possible to fashion some device that harnesses that same force which causes tidal changes and whatnot into a conventional energy you could use to power computers and heat homes?

Indirectly, yes. Tidal power is being harnessed in many parts of the world - it should be a big thing soon. In Ireland and the UK, we are very well placed to profit from this source of power - it's a pity more support is not forthcoming.

Link

**Kralizec** · 06-14-2006, 01:58

Originally Posted by Tiberius

It's impossible to ignore gravity though, and the movement will inevitably cause heat, causing it to lose heat and therefore eventually run out of energy.

Perpetual motion is impossible.

The gravity is what makes it circle. I should have added "in a theoretical vacuum". A better way to put is that in a vacuum, an object that is once propelled will keep moving forward. A space ship traveling through vacuum doesn't need fuel to move forward, just to accelerate and decellerate. A perpetuum mobile is theoreticly possible in vacuum (therefore absolutely impossible in practice), but as said before will never be usuable as an energy source.

**Uesugi Kenshin** · 06-11-2006, 05:34

Originally Posted by Kralizec

If I remember right, there's one (theoretical) instance of perpetual motion: when one celestial object circles around the other, ignoring the gravitational pul from other objects. Since there's no friction of atmosphere, it could move on forever- it's impossible to make it into an energy source though, because that would slow down the orbit.

Not correct. There are some particles scattered through the near vaccum of space, so even then the friction caused by these particles would slow you down. This is pretty negligible most of the time, but if you're talking about a long enough period of time then it can cause quite a significant effect and would eventually stop the object.