Question About Perpetual Motion

[Jill BioSkop]

So I found this puzzle in a book:
You have two adjacent frictionless planes with a frictionless chain resting on them. One is at a smaller angle from the vertical axis than the other, and this one has less links of the chain resting on it (see fig. 1).
Fig. 1 : (Please forgive crappy mouse drawings.)

Does the chain slide?

#Answer given in the book:
No, because: suppose you link the chain with itself to form a complete circle (see fig. 2), then if the chain slides it will cause perpetual motion to occur. And perpetual motion doesn't exist, hence the chain doesn't slide.
Fig. 2 :

#My beef with the above answer:

Perpetual motion: "motion that continues indefinitely without any external source of energy; impossible in practice because of friction" (wordnetweb.princeton.edu/perl/webwn)
"The most commonly contemplated type of perpetual motion machine is a mechanical system which (supposedly) sustains motion indefinitely, despite losing energy to friction and air resistance. This violates the first law of thermodynamics (conservation of energy)."(http://en.wikipedia.org/wiki/Perpetual_motion)

Other definitions seem to refer to machines that produce more energy than they use. (Eg: overunity devices (referring to devices with an energy efficiency greater than 1.0) (http://en.wikipedia.org/wiki/Perpetual_motion))

From this:
The biggest cause of energy loss in machines is friction. The above hypothesis uses frictionless objects. This removes the biggest obstacle to perpetual motion. If one follows the 'frictionless' criteria to its logical consequences, then the answer given rests on an incoherent argument. Of course, other sources of energy loss could be air/gas around the setup for eg.

Suppose the planes are set up as in fig. 2; then the weight of the chain could pull the whole of it down on one side (down the side which, once you take in account the weight of the links and the support given by the planes, weighs heaviest). However this implies gravity, and gravity can cause loss of energy, but couldn't the part of the chain being pulled down compensates exactly for the part of the chain being pulled up? Couldn't there be, not new energy being made to compensate, but perfect conservation of energy to start with?

I think the answer to the original question depends on the proportion of links on either plane (if there is no friction plane length matters much less, if at all), and hence that there are insufficient data to answer the question.

Agree/disagree? Thoughts?

 

[ApostateAbe]

I think you are perfectly right, and you had best write a complaint letter. All you need to do is to imagine a difference in angles and weights such that one side will most certainly pull the other side off the planes. If you have a loop instead of two loose ends, then it changes the dynamic, because resistance will be applied mutually from both sides, and you will have equilibrium and no movement regardless of angles or weights.

 

[Vrecknidj]

Why would a frictionless chain exist? How are frictionless planes adjacent?

I don't accept the axioms.

Dave

 

[warryer]

This wouldn't be able to start on its own. If it was an open chain it could but, your chain will cancel out ALL horizontal tension because it is linked to itself. You would only be left with the downward pull of gravity. Draw a free-body and see.

However if you were to give it a push.... with no where else for the energy to go, the kinetic motion would be constant forever.

 

[dreamoftheunknown]

Wow! I'm impressed by this thread. I agree that the answer in the book is not logically sound. I kind of take issue to the usage of a chain in the question, as well, since I think it could be a bit misleading (due to the discreteness of the elements of a chain). It would have been better formulated as a rope problem (well, an incompressible/unstretchable rope). As a rope, it becomes clearer that the forces between infinitesimal rope elements must balance or else the rope would break.

 

[Allinea]

The more I think about it, frictionless things don't exist, and never will. Thus, this problem is either impossible to answer or is extremely hard.

 

[Latro]

The more I think about it, frictionless things don't exist, and never will. Thus, this problem is either impossible to answer or is extremely hard.

No, this isn't the case. With the Newtonian axioms friction is just another force, which could be present or not. The fact that it doesn't agree with thermodynamics has nothing to do with anything.

Incidentally, the perpetual motion argument is useless if friction is assumed to be zero; in a frictionless system the inertia of a moving object keeps it in motion indefinitely.

 

[Melllvar]

I suppose by that logic no chain that is connected in a circle will ever slide. They're really changing the situation by adding the connecting chain at the bottom. In the first scenario it's already in equilibrium as long as the chain is the same vertical length from the top of the triangle on both sides, and in the second it is in equilibrium only when the bottom is hanging in the position that minimizes it's potential energy.

Suppose the chain has heavier links on one side than the other. Then it would slide, linked or not, until it found a new equilibrium. The explanation would imply otherwise. It's not a good answer IMO.

 

[Melllvar]

Suppose the planes are set up as in fig. 2; then the weight of the chain could pull the whole of it down on one side (down the side which, once you take in account the weight of the links and the support given by the planes, weighs heaviest). However this implies gravity, and gravity can cause loss of energy, but couldn't the part of the chain being pulled down compensates exactly for the part of the chain being pulled up? Couldn't there be, not new energy being made to compensate, but perfect conservation of energy to start with?

I think the answer to the original question depends on the proportion of links on either plane (if there is no friction plane length matters much less, if at all), and hence that there are insufficient data to answer the question.

Agree/disagree? Thoughts?

I should have read your actual question before posting. There is sufficient data, and in Fig. 1 it won't slide as long as the chain links are of the same weight, length, and each side reaches down the same vertical distance. If the angles are both zero then the chain lengths are the same, weights are the same, and it's in equilibrium. As the angles change the inclined plane compensates for different length/weight of chain, since part of gravity is acting into the plane. In the case that one side of the plane is level all of gravity is acting into the plane, so an infinite length of chain would be needed. You can show this all rather quickly with Newtonian diagrams.

If it started in motion, with no resistance to slow it down though, then it would continue it's motion indefinitely. This would be true too if the bottom part started out of equilibrium. I misspoke in my last post, it would not find a new equilibrium since there is no energy loss in the system.

Also, I should point out that energy isn't "lost" to gravity, it just gets converted from one form to another, like potential to kinetic in a falling object. You're right that the part being pulled down would compensate for the part being pulled up; the falling side converts the same amount of potential energy to kinetic as the rising side does from kinetic back to potential in any given moment of time.

 

[Hawkeye]

I return to this! :O

This has to be one of the daftest questions I've seen in a long time. The answer does not address the said chain but instead conjures up another chain that reacts in a totally different way.

The answer also contradicts the question... If you have two frictionless surfaces in contact with each other then perpetual motion is possible. It is only possible this way...

The writer has applied real world physics to an imaginary scenario that was created to go against reality. Basically... this person is a plonker and should seriously think about a career change; perhaps a dragon slayer or vampire...



The correct answer to the question is simple: Yes, unless the chain is in equilibrium in which case it would remain stationary.

 

[Jill BioSkop]

Thanks for the all feedback.

 

[Latro]

Gravity is a conservative force, incidentally; the kinetic energy it imparts to an object was potential energy before.

 

[fullerene]

And for people saying a frictionless environment is impossible: what about space? Space isn't even 100% fricionless, but it's pretty damn close, and I don't think anyone would argue that it seems as if, without friction, motion becomes closer to perpetual motion.

yeah... that is a ridiculously stupid explanation for severa reasons... but I think all of them have been covered here by various people.

 

[nexion]

And for people saying a frictionless environment is impossible: what about space? Space isn't even 100% fricionless, but it's pretty damn close, and I don't think anyone would argue that it seems as if, without friction, motion becomes closer to perpetual motion.

yeah... that is a ridiculously stupid explanation for severa reasons... but I think all of them have been covered here by various people.

Yep. Space is virtually frictionless, such that acceleration does not slow down as time passes. Simply amazing.

 

[babrock]

Warryerr is correct on this.

 

[bumsyspin]

The more I think about it, frictionless things don't exist, and never will. Thus, this problem is either impossible to answer or is extremely hard.

You're here and you don't like thought experiments?!

 

[SpaceYeti]

Yes, this situation is obviously silly. If you grant the situation no friction, then friction would obviously not be a concern.

Incidentally, I have an idea for a near perpetual motion. I keep meaning to email physicists about it, but I always forget to figure out who I'd send it to. I'm not telling it to you guys, It's my baby.

 

[nexion]

Yes, this situation is obviously silly. If you grant the situation no friction, then friction would obviously not be a concern.

Incidentally, I have an idea for a near perpetual motion. I keep meaning to email physicists about it, but I always forget to figure out who I'd send it to. I'm not telling it to you guys, It's my baby.

There's no such thing as a near perpetual motion machine. Even if you were to make something that provided energy for 100,00 years without any further stimulus, 100,000 years is nothing in the face of eternity.

Cool idea, though. Maybe someday we shall hear about the near infinite (lol, sorry) energy source that <insert name here> created, and I will say, "Dude, I knew that guy. Lucky bastard."

 

[commandolam]

Atoms are in perpetual motion, they will never stop moving.

/thread?

 

[bumsyspin]

Atoms are in perpetual motion, they will never stop moving.

/thread?

Oh they will.

 

[SpaceYeti]

There's no such thing as a near perpetual motion machine. Even if you were to make something that provided energy for 100,00 years without any further stimulus, 100,000 years is nothing in the face of eternity.

Cool idea, though. Maybe someday we shall hear about the near infinite (lol, sorry) energy source that <insert name here> created, and I will say, "Dude, I knew that guy. Lucky bastard."

Then I'll say it this way; I have an idea for a machine designed to negate it's own friction as much as possible. Too bad that's all it can do.

 

[babrock]

Oh they will.

I clicked on this and saw that it was reffering to how because of dark energy t universe will continue to expand untill t average kinetik energy per unit of volume aproaches zero, meaning things will eventualy gets quite cold in our universe.

This is only due tho to how all matter will continue to travel away from each other involving motion of yet another variety. This tho is due to another force acting on it. This force, dark energy is quite likely coming from another universe. In any event, it is not at all well understood or another dimension and where it is coming from is not known.

 

[avanover]

Perpetual motion does exist. The motion/energy is merely released as heat due to friction. The motion merely continues throughout external atoms in a diffusion and energy meets equilibrium but is nevertheless perpetual. To have perpetual motion occur in a contained environment for practical use, you would need a frictionless environment.

 

[Vrecknidj]

This is an interesting question: "Is the universe a perpetual motion machine?"

Based on the previous post, I suppose the universe's friction would have to be zero -- whatever that means.



Dave

 

[walfin]

Well since the diagram is not drawn to scale, it could slide until it finds equilibrium.

No, it wouldn't reach perpetual motion, because that system would reach an equilibrium point eventually. So the answer seems wrong.

 

[avanover]

This is an interesting question: "Is the universe a perpetual motion machine?"

Based on the previous post, I suppose the universe's friction would have to be zero -- whatever that means.



Dave

The universe is, indeed, a perpetual motion machine. Friction within the system does not matter, it's external friction which we are worried about because energy is sent off into another system. Supposing that this is the only universe, we needn't worry about friction. The only way a universe could stop motion is if the atoms reached a temperature of 0 Kelvin, which is said to be impossible.

 

[gruesomebrat]

Alright, so I know that digging up old posts is sort of frowned upon, but I really didn't see the point of opening a new thread to ask a question regarding perpetual motion when this one was sitting here, unused.

I was over on Wikipedia earlier today, looking at their page on the subject, but unfortunately, it seems that this is one thing that Wikipedia actually confuses me further on. I have two major problems with the article, but I'm not sure who's at fault; me, the article, or the scientific community.

First, the article seems to be saying that only a perpetual motion machine in an isolated system violates the first and/or second law of thermodynamics. The problem is that the wiki for "Isolated system" says that isolated systems are purely theoretical, and cannot exist IRL, with the possible exception of the universe as a whole. If this is true, then the question arises as to whether it is only perpetual motion machines in an isolated system that are impossible. If so, does that mean that they may still be possible in, for example, a closed system, where matter cannot enter or leave but energy can?

It seems to me that this article, if it properly portrays the scientific community's thoughts on perpetual motion, opens up the possibility of an inventor coming up with a machine that converts mechanical energy back and forth between its forms in such a way as to be useful in electric generation.

The second problem I have with this article is the reference to scientific consensus. Apparently, a scientific consensus is only "the collective judgment, position, and opinion of the community of scientists in a particular field of study", and "implies general agreement, [but] not necessarily unanimity". If I'm reading this correctly, that means that the statement that "There is a scientific consensus that perpetual motion in an isolated system would violate the first law of thermodynamics and/or the second law of thermodynamics" essentially means nothing. The combination of scientific consensus only being an agreement by those in the field, and the fact that isolated systems are purely theoretical, translates the above statement into something like "We are pretty sure that perpetual motion can't be done in a theoretical world."

Does this mean that scientists looked at the problem of perpetual motion, said 'No-one so far has managed to do it, so it can't be possible and we're not even going to bother", and just walked away? Does that mean that I could still build a perpetual motion machine, market it as a zero-emission engine, and make billions as a new automotive manufacturer? Or is there a flaw somewhere in my logic regarding the article's claims?

 

[walfin]

Won't linking the chain together change the forces on the ends of the chain? The linking chain will exert forces on both sides due to its weight.

 

[Words]

A closed system within the universe where matter cannot leave or enter? Don't think that's possible in the quantum level, with the supposed interconnected and simultaneous realigning of electrons[which can be classified as matter] between systems.