Why Is Perpetual Motion Impossible?

[Caramon]

Hey,
Is the only reason for a perpetual motion machine being impossible, both in theory and in practice, simply due to friction? Because there are substances, such as superfluids, and potentially other forms of matter that could be discovered in the future where there is zero friction, so would it be possible, simply in principle, to create a perpetual motion machine using these substances?

 

[Pinu7]

Perpetual motion is a misnomer. It means a system that can continuously give off energy ad infinitum. Of course, this violates the conservation of energy and is therefore impossible.

However, this does not stop people from selling "perpetual motion electric generators" to the uneducated.

 

[Caramon]

Why does it violate the conservation of energy? Could a perpetual motion machine not be created that converts one form of energy into another by the use of a superfluid, so there is no loss of energy to the environment through friction...?

 

[Drakkith]

Why does it violate the conservation of energy? Could a perpetual motion machine not be created that converts one form of energy into another by the use of a superfluid, so there is no loss of energy to the environment through friction...?

There are several ways for a system to lose energy, only one of which is friction. Even IF you could make eliminated all losses you would merely make a very efficient machine. IE you would get 100% of the energy input into the system as output as some type of work. But guess what, you still have to input energy. You can't make a free energy generator or anything like that because you have to have some sort of energy input to get anything out.

As to the why, it is simple. Because nature has shown us that it works that way and no way else.

 

[czelaya]

I always kept thinking that a perpetual motion was plausible in some sort of manner.

2 semesters of thermodynamics at the undergraduate level and 3 semesters of graduate thermodynamics and statistical mechanics and alas you know why it's immposible.

Thermodynamics is key to understanding this:

http://en.wikipedia.org/wiki/Perpetual_motion

I suggest understanding a Carnot cycle first (understand thermodynamic parameters are such as internal energy, adiabatic process, work, enthalpy, and isenthalpic mean first):

http://en.wikipedia.org/wiki/Carnot_cycle

 

[Drakkith]

Yes, I have never understood why people think that Perpetual Motion and Free Energy machines are possible. (At least with any near-moderate future technology. But let's keep Sci-Fi out of this)
Energy cannot be made from nothing, only transferred from something else.

 

[russ_watters]

Why does it violate the conservation of energy? Could a perpetual motion machine not be created that converts one form of energy into another by the use of a superfluid, so there is no loss of energy to the environment through friction...?

It has nothing to do with friction. If you have a pile of apples and you take apples from that pile, can you take apples away from it forever? Of course not - eventually you will run out of apples. So to with energy: a system has only a certain amount of energy in it, so you can't continually extract energy without eventually running out.

 

[waynexk8]

Well the Earth goes around the Sun, and so on. And just look at all the other things in the Universe that are the same as that, they go on and on, thus thay are perpetual motion.

Wayne

 

[JaredJames]

Well the Earth goes around the Sun, and so on. And just look at all the other things in the Universe that are the same as that, they go on and on, thus thay are perpetual motion.

Uh, no. They most certainly are not.

Eventually our sun will use up all its fuel and die. That will radically alter things for a start - no more energy for us. The Earth, Moon, Mars etc orbits are all degrading in some way and won't stay as they are forever.

The universe will also suffer a form of death (heat death I believe).

They will not "go on and on". Don't confuse going on for a long time as something being perpetual.

 

[Rap]

If you define perpetual motion as motion that does not stop, then the Earth going around the sun is perpetual motion, practically. But the usual definition of perpetual motion is a machine that provides an endless source of energy, which, by the conservation of energy, is impossible.

 

[RandomGuy88]

Well the Earth goes around the Sun, and so on. And just look at all the other things in the Universe that are the same as that, they go on and on, thus thay are perpetual motion.

Wayne

Something that simply moves forever is not what is meant by a perpetual motion machine. Perpetual motion is referring to something that can give off useful energy forever.

 

[JaredJames]

If you define perpetual motion as motion that does not stop, then the Earth going around the sun is perpetual motion, practically.

The definition of perpetual is something that goes on forever. As such, the Earth around the sun is not perpetual motion - it just lasts a very long time.

 

[russ_watters]

There is no need to have the discussion of what the difference between "perpetual motion" and "perpetual motion machine" is (or hairsplit about whether or not an orbit is perpetual motion) - the OP said in the OP "perpetual motion machine".

 

[uart]

Something that simply moves forever is not what is meant by a perpetual motion machine. Perpetual motion is referring to something that can give off useful energy forever.

Literally however "perpetual motion" does in fact just mean something that moves forever. I believe the OP is correct in saying that approaching this as a limit point violates no physical laws. (By limit point I meant that it's the point which separates what is physically possible from what is not).

The problem is really just one on nomenclature. Over time the term perpetual motion has come to mean more than just motion which never ends, it often carries the implication that we can extract energy from this motion and it will still go on forever. Obviously that definition of perpetual motion clearly does violate physical laws.

I guess the term "perpetual motion" has come to have this second meaning because historically almost every proponent of perpetual motion machines has sort to use it to that end.

 

[DixitDominus]

If we can eliminate friction, then it is possible.

In frictionless void space a couple objects can easily rotate around their common center of mass, much like the Moon goes "around" the Earth.

However the original problem was to create such a device on Earth, where we have an atmosphere, air resistance and friction everywhere.

Extraction of energy from such a device is another thing though, if we can make a non friction device that will operate on Earth, extracting energy from such a device will cause it to slow down.

 

[JaredJames]

If we can eliminate friction, then it is possible.

No, if we can eliminate losses - it's not only friction.

In frictionless void space a couple objects can easily rotate around their common center of mass, much like the Moon goes "around" the Earth.

Only as long as no external forces act on them - if we started to extract the orbital energy they have then we'd be providing that external force (or resistance to motion) and as such would affect the orbits.

However the original problem was to create such a device on Earth, where we have an atmosphere, air resistance and friction everywhere.

The problem never states where the creation has to take place - this is irrelevant in any case.

Extraction of energy from such a device is another thing though, if we can make a non friction device that will operate on Earth, extracting energy from such a device will cause it to slow down.

Again, it is independent of location.

You can only extract the energy the system has - once it's gone it's gone.

 

[sophiecentaur]

"Goes on for ever"
Sounds like the endless stream of suggestions that perpetual motion is a goer.

 

[JaredJames]

"Goes on for ever"
Sounds like the endless stream of suggestions that perpetual motion is a goer.

"Goer"?

 

[sophiecentaur]

English vernacular, I'm afraid, for something which will 'go', work or be feasible.
It's in there amongst other definitions if you google it.
I take it you are not from the UK?

 

[DixitDominus]

My understanding of this very old problem of Perpetual Motion is that people tried to create a real device that does that. On Earth, this is where they lived and had access at the time and this place posed some restrictions because of friction, air resistance etc.

If they had access to space, as we do, then they would have easily created something that moves without stopping, however the understanding at the time was that space was a place for Gods only.

If we generalize the problem, then location is irrelevant. I do not thing that old timers looked at the problem this way, they were talking about a real device on Earth, not a device somewhere among the stars.

 

[JaredJames]

English vernacular, I'm afraid, for something which will 'go', work or be feasible.
It's in there amongst other definitions if you google it.
I take it you are not from the UK?

I'm from Wales.

I've only ever heard it in reference to girls.

Number 3 of the definitions there: http://en.wiktionary.org/wiki/goer

 

[sophiecentaur]

Oh yes:
'bit of a goer - know what I mean, know what I mean?'
Monty Python: nudge nudge sketch.

 

[JaredJames]

Oh yes:
'bit of a goer - know what I mean, know what I mean?'
Monty Python: nudge nudge sketch.

Hence my confusion, although I'm sure many wish they had a 'perpetual' girl, if you know what I mean.

 

[sophiecentaur]

My understanding of this very old problem of Perpetual Motion is that people tried to create a real device that does that. On Earth, this is where they lived and had access at the time and this place posed some restrictions because of friction, air resistance etc.

If they had access to space, as we do, then they would have easily created something that moves without stopping, however the understanding at the time was that space was a place for Gods only.

If we generalize the problem, then location is irrelevant. I do not thing that old timers looked at the problem this way, they were talking about a real device on Earth, not a device somewhere among the stars.

One of the big steps was to realize that the same Physics works out there as works down on Earth. Beginnings of the Age of Enlightenment.

 

[DixitDominus]

Physics is the same, the environment in which the device must operate is not.

There is no air resistance in void space, that is because there is no air there, not because we need to apply a different kind of Physics there.

So, a device that can stop when operating on Earth, will keep moving in void space if the reason for stopping is air resistance.

If old timers had access to void space the Perpetual Motion thing would be considered solved long ago, and this would not require that some new Physics would have to be discovered to achieve that.

 

[JaredJames]

If old timers had access to void space the Perpetual Motion thing would be considered solved long ago, and this would not require that some new Physics would have to be discovered to achieve that.

Perpetual motion and it's associated devices was solved a long time ago - no new physics were need to do so.

Basic thermodynamics tells you it doesn't work.

The problem is we humans don't like it when the rules say something is impossible. We have an arrogance that makes us believe "anything is possible". Hence the continued debate on the matter.

 

[Buckleymanor]

Perpetual motion and it's associated devices was solved a long time ago - no new physics were need to do so.

Basic thermodynamics tells you it doesn't work.

The problem is we humans don't like it when the rules say something is impossible. We have an arrogance that makes us believe "anything is possible". Hence the continued debate on the matter.

We live in a universe that started 15 billion years ago from a quantum fluctuation now if that is not a free lunch I don't know what is,where is the arrogance in in believing anything might be possible.Why should we accept the impossible when we are surrounded by evidence that shows the incredible possible.
Ok. it might not be perpetual but we are not going to be around to say it is not.

 

[JaredJames]

We live in a universe that started 15 billion years ago from a quantum fluctuation now if that is not a free lunch I don't know what is,

You assume the energy came from no where? Who knows? Could be right, could be wrong. The beginning is a tricky subject at best.

where is the arrogance in in believing anything might be possible.

Because we have no logical reason to do so - if everything about it tells you it's a duck, why would you go about preaching it's a chicken? I've given many examples here on PF regarding the impossible and it is perfectly clear things are impossible. No amount of wishful thinking will change that.

Why should we accept the impossible when we are surrounded by evidence that shows the incredible possible.

I think you'll find all the evidence says that free energy is impossible.

We are surrounded by things that are incredibly unlikely - life itself - but then given the scale of the universe the odds can actually be shown to be in favour there being life at least once out there. Like the lottery, the odds are against you winning - 1 in 14 million in the UK - but if there are more than a certain number of tickets sold the odds of someone winning are actually quite reasonable.

So in that respect, there is very little within our universe that shows evidence of the impossible - just the highly implausible.

Ok. it might not be perpetual but we are not going to be around to say it is not.

The fact we are not around does not change our observations.

 

[Buckleymanor]

You assume the energy came from no where? Who knows? Could be right, could be wrong. The beginning is a tricky subject at best.

Agreed

Because we have no logical reason to do so - if everything about it tells you it's a duck, why would you go about preaching it's a chicken? I've given many examples here on PF regarding the impossible and it is perfectly clear things are impossible. No amount of wishful thinking will change that.

Absolutely

I think you'll find all the evidence says that free energy is impossible.

Tell the universe it is not so.

We are surrounded by things that are incredibly unlikely - life itself - but then given the scale of the universe the odds can actually be shown to be in favour there being life at least once out there. Like the lottery, the odds are against you winning - 1 in 14 million in the UK - but if there are more than a certain number of tickets sold the odds of someone winning are actually quite reasonable.

My ramberlings were supposed to be inclusive of the universe as a whole including life, what are the odds on that!

So in that respect, there is very little within our universe that shows evidence of the impossible - just the highly implausible.

If it's only implausible perpetual motion must be a dodle.

The fact we are not around does not change our observations.

If we are not around there are no observations.

qqqq

 

[JaredJames]

1. The universe tells us it is not possible.

2. My point was to demonstrate that nature isn't proving the impossible when it comes to life - only something implausible. Your "nature shows the impossible is possible" idea is ridiculous.

I didn't say PMM is implausible.

There is a difference between something highly unlikely happening - life - and something being impossible. Don't confuse the two.

Winning the lottery = highly unlikely. Continuing my life as it is now without a brain = impossible.

One can occur, one cannot. Perpetual energy is the latter.