Perpetual motion machine - Intuitive meaning

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I'm reading the first Feynman volume on physics.
In the chapter about Conservation of Energy, the author explains with the following sentence what is a perpetual machine ( in this case a weight-lifting machine).

"If, when we have lifted and lowered a lot of weights and restored the machine to the original condition, we find that the net result is to have lifted a weight, then we have a perpetual motion machine because we can use that lifted weight to run something else."

I cannot completely grab the meaning of the phrase. Especially the bold sentence. What Feynman means in saying "we find that the net result is to have lifted a weight"? What happens in a perpetual weight-lifting machine ?
Can you help me ?
 

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  • #2
Borg
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Links to the articles that you have a question about would help. I assume that this is the correct lesson?
http://www.feynmanlectures.caltech.edu/I_04.html

Consider weight-lifting machines—machines which have the property that they lift one weight by lowering another. Let us also make a hypothesis: that there is no such thing as perpetual motion with these weight-lifting machines. (In fact, that there is no perpetual motion at all is a general statement of the law of conservation of energy.) We must be careful to define perpetual motion. First, let us do it for weight-lifting machines. If, when we have lifted and lowered a lot of weights and restored the machine to the original condition, we find that the net result is to have lifted a weight, then we have a perpetual motion machine because we can use that lifted weight to run something else. That is, provided the machine which lifted the weight is brought back to its exact original condition, and furthermore that it is completely self-contained—that it has not received the energy to lift that weight from some external source—like Bruce’s blocks.
As he states, if you see the word perpetual used with the description of any kind of device, you should assume that it doesn't exist. The paragraph above defines a device that cannot exist and the things that must be considered in defining a perpetual motion machine. Very often, people mistakenly think that they have created some sort of device that "runs forever" without supplying any energy. However, the three keys items above (that I highlighted) are where everyone eventually fails with respect to at least one of them.
 
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What Feynman means in saying "we find that the net result is to have lifted a weight"?
Net result means that any other temporary changes were reversed. If a lever was pushed in the process then the lever is reset to its original position. If a spring was decompressed, it is recompressed.

So a perpetual motion machine is one where the machine can be reset to its original state and the only other effect is to have lifted a weight. I.e. no fuel refilled or battery recharged, etc.
 
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  • #4
CWatters
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Might help to understand conservative forces.

https://en.m.wikipedia.org/wiki/Conservative_force

In short, If you move something around a closed path (eg a path that returns to its starting position) then the net work done by a conservative force is zero.

So if an ideal (eg frictionless) robot climbed a mountain and then returned to the starting point it would have done no net work against gravity and gravity would do no net work on the robot. It could arrive back with it's batteries recharged to the same state as when it left by its regenerative braking system. It does not matter which route the robot takes either. It can go up one way and come back another and the result would be the same.

That Feynman quote is essentially saying that if the ideal robot somehow managed to return to the starting position in the same condition it left (eg fully charged) AND had done net work by raising a weight (eg leaving a weight at the top) then it would be a perpetual motion machine because it had created energy. In practice if the robot left a weight at the top there would less potential energy harvested on the way back down so it wouldn't arrive back in exactly the same condition.

There are many so called free energy machines on YouTube that claim to use gravity or magnets, which are conservative forces in this context. However they almost all rotate meaning they return to their starting position once per revolution. That's a good clue they don't work.

Their inventors try to make them more and more complicated but remember...the path doesn't matter. If it ever returns to the same position/condition the net work done is zero.
 
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Net result means that any other temporary changes were reversed. If a lever was pushed in the process then the lever is reset to its original position. If a spring was decompressed, it is recompressed.

So a perpetual motion machine is one where the machine can be reset to its original state and the only other effect is to have lifted a weight. I.e. no fuel refilled or battery recharged, etc.
If the machine returns to the initial conditions with no energy loss, why it's generating energy ?
Is it a perpetual machine a machine where the initial energy is mantained ?
 
  • #6
jbriggs444
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If the machine returns to the initial conditions with no energy loss, why it's generating energy ?
Is it a perpetual machine a machine where the initial energy is mantained ?
Not only did the machine return to its initial state. It did some useful work on the environment.

Edit to add:

Wikipedia summarizes the classifications of perpetual motion machines into three kinds. The first kind generates energy from nowhere. The second kind generates useful work from thermal energy. The third kind just keeps going and going, neither producing nor consuming energy -- e.g. a frictionless wheel.

We're not talking about the third kind.
 
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Not only did the machine return to its initial state. It did some useful work on the environment.
Hi, if a perpetual pendulum starts at a certain position and the return in the same position, doesn't it return to the initial energy state ? So, how it generates energy ?
 
  • #8
jbriggs444
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Hi, if a perpetual pendulum starts at a certain position and the return in the same position, doesn't it return to the initial energy state ? So, how it generates energy ?
That is a perpetual motion machine of the third kind -- frictionless. Not what Feynman was talking about. [See edit to post #6 above]
 
  • #9
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Not a perpetual motion machine, but an electric mining truck with 65 ton payload that will pump 200 Kwh of excess electric power into the utility grid every night. https://phys.org/news/2017-09-e-dumper-world-largest-electric-vehicle.html. They will do it by loading the truck at the top of the hill, driving down using regenerative braking, then driving uphill empty. The uphill trip is expected to use 10 Kwh less electricity than is generated on the downhill trip, so they have to partially discharge the battery every night.
 
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Not only did the machine return to its initial state. It did some useful work on the environment.

Edit to add:

Wikipedia summarizes the classifications of perpetual motion machines into three kinds. The first kind generates energy from nowhere. The second kind generates useful work from thermal energy. The third kind just keeps going and going, neither producing nor consuming energy -- e.g. a frictionless wheel.

We're not talking about the third kind.
Is it the feynman weight-lifting machine a perpetual machine of first type?
 
  • #11
jbriggs444
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Is it the feynman weight-lifting machine a perpetual machine of first type?
Yes.

Perhaps I have misunderstood your question. You want to know how a perpetual machine of the first kind works? The answer is that it does not. No such machine is possible. No such machine exists.
 
  • #12
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It sounds to me like @Aleoa just has an issue with Feynman's wording. As written, it sounds like a self-contradiction to me too. So I would just suggest to ignore that sentence about returning to its original configuration and instead say it returns to a configuration with more potential energy than it started with (one weight higher than it was before).

[edit]
It could also help to describe in more detail:

You have a machine with four wheels, of different sizes. The wheels are connected via a complex system of gears. Attached to each of the wheels is a different weight. One weight is located each at 12:00, 3, 6 and 9 on their respective wheels. You release the machine to turn and it makes a lot of noise, steam comes out, music plays, bright lights flash, and you taste cinnamon in our mouth*. At the end of all this the machine stops and the first three weights return to 12, 3 and 6 on their wheels, but the 4th weight is now at 12:00. The machine has broken the first law of thermodynamics; it is a perpetual motion machine of the first kind.

*Point is: Don't let all this nonsense distract you. It doesn't matter. Only the starting and ending points matter.
 
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  • #13
Ibix
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I agree with Russ that the paragraph as quoted is confusingly written. But what Feynman has in mind is something like this famous Escher drawing:
https://en.wikipedia.org/wiki/Waterfall_(M._C._Escher)#/media/File:Escher_Waterfall.jpg
The "weight being lifted" is the water flowing along the channel from the bottom of the fall to the top. The energy of the fall turns the water wheel, which can drive machinery, before flowing back "up" the channel. In this case there isn't really a reset - the machine is always in the same state, although one could imagine a rubber duck floating around so you could track a "bit" of water round the loop and regard it as reset when the duck returns to its start position. (As an aside, I recall a short story in which an inventor builds one of these with a generator and connects it to an electric dark bulb, plunging a room into eternal darkness).

Escher is, of course, playing games with perspective and the machine can't actually be built (although you can fake it with some use of forced perspective). "Real" perpetual motion machines have more subtle flaws. Some people design them as "spot the flaw" challenges knowing that they don't work; others have simply outwitted themselves.
 
  • #14
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As written, it sounds like a self-contradiction to me too. So I would just suggest to ignore that sentence about returning to its original configuration and instead say it returns to a configuration with more potential energy than it started with (one weight higher than it was before).
Hmm, as I read it the weight is not a part of the machine. So the machine returns to its original state and the only effect on the environment is to lift the weight. But I could be misconstruing it.
 
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  • #15
russ_watters
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Hmm, as I read it the weight is not a part of the machine. So the machine returns to its original state and the only effect on the environment is to lift the weight. But I could be misconstruing it.
No, I'm sure you are correct: what you said is the interpretation that results in it being internally consistent and an accurate description of reality (it's easy to interpret the question correctly when you already know the answer!). I'm just saying I see why it could be confusing to some people: by using weights as what's inside and outside the device, it is easy to confuse them with each other. Grammatically, it is difficult to distinguish the "weight" in the second half of the sentence from the "weights" in the first half. I think it would have been clearer if, for example, the machine used weights and the output was filing a reservoir.

Ironically, whether the other weight is inside or outside the box, it doesn't actually change anything about the science, it just changes the description.
 
  • #16
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I see why it could be confusing to some people:
Yes, I see it now too. Feynman is usually quite clear. Must have been an off day
 
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Schermata 2018-04-03 10:36:10.png

Feynman says:
"A very simple weight-lifting machine is shown in Fig. 4-1. This machine lifts
weights three units “strong.” We place three units on one balance pan, and one
unit on the other
."

Is the picture wrong ? Shouldn't the 3 weights be on the right pan and the unit weight on the left pan ?
Feynman is talking about a lever that lifts three units of mass with 1 unit of mass...
 

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Is the picture wrong ? Shouldn't the 3 weights be on the right pan and the unit weight on the left pan ?
The picture is correct. Levers work as drawn
 
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Suppose we have an ideal balanced lever. If this system is reversible then any infinitesimal weight put on one end of the lever will make the other end to get down . Is it a intuitive definition of reversible system ?
 
  • #20
russ_watters
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Suppose we have an ideal balanced lever. If this system is reversible then any infinitesimal weight put on one end of the lever will make the other end to get down . Is it a intuitive definition of reversible system ?
I don't understand what you are trying to say: if you put a weight on a lever, the side you put the weight on goes down. The other side goes up.
 
  • #21
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I'm stuck with this part of the lecture:
"A very simple weight-lifting machine is shown in Fig. 4-1. This machine lifts
weights three units “strong.” We place three units on one balance pan, and one
unit on the other. However, in order to get it actually to work, we must lift a
little weight off the left pan. On the other hand, we could lift a one-unit weight
by lowering the three-unit weight, if we cheat a little by lifting a little weight
off the other pan."

I'm not able to image how this machine work, and how it's connected with the lever picture i previously posted...
 
  • #22
jbriggs444
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I'm not able to image how this machine work, and how it's connected with the lever picture i previously posted...
The three weights are on a short arm -- three units long. The one weight is on a long arm -- one unit long. The two arms are in balance. If, as pictured, the three weights are low and the one weight is high then a feather added on top of the one weight will cause the three weights to be lifted. Of course, the one weight and the feather will fall at the same time. Then if the feather is moved on top of the three weights, it will cause the one weight to rise and the three weights to fall.

It is just that simple.
 
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  • #23
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The three weights are on a short arm -- three units long. The one weight is on a long arm -- one unit long. The two arms are in balance. If, as pictured, the three weights are low and the one weight is high then a feather added on top of the one weight will cause the three weights to be lifted. Of course, the one weight and the feather will fall at the same time. Then if the feather is moved on top of the three weights, it will cause the one weight to rise and the three weights to fall.

It is just that simple.
But, does the picture i posted represent a lever that is moving ?
In fact, isn't a steady and balanced lever perpendicular to the ground ? Like this picture:

index.png


Furthermore, this is an unstable equilibrium point, since if i put a feather on one end of the balanced lever, this end starts to go down until it's stopped by the ground. And this should happen in the lever system described by Mr. Feynman. Isn't it ?
 

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jbriggs444
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But, does the picture i posted represent a lever that is moving ?
In fact, isn't a steady and balanced lever perpendicular to the ground ?
What point are you trying to make? One can have a balanced lever that is stationary in any orientation. In any case, such fine distinctions are irrelevant to the point that Feynman is attempting to convey.
 
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  • #25
CWatters
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My translation..

"A very simple weight-lifting machine is shown in Fig. 4-1. This machine lifts
weights three units “strong.” We place three units on one balance pan, and one
unit on the other.
So it starts off balanced. Not moving. Not "working".

However, in order to get it actually to work, we must lift a
little weight off the left pan.
You can make it work by making the three unit side slightly lighter. Now there is a net clockwise torque on the balance.

On the other hand, we could lift a one-unit weight
by lowering the three-unit weight, if we cheat a little by lifting a little weight
off the other pan."
Or you can lift the one unit weight by making the one unit side slightly lighter. So there would be a net counter clockwise torque.
 
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