Feynman's reversible and irreversible machines

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TL;DR Summary

Feynmans reversible and irreversible machines

I started reading "Feyman Lectures on physics" and stuck on his explanation about reversible and irreversible machines. I've tried to read other answers to this question, but I couldn't get the point. Here is the first thing:

"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. "

I image myself a see-saw (weight-lifting) with some weight A on one side, which will be lifted by putting some weight B (by myself) on otherside. Then when I take away weight B from the see-saw, clearly the weight A will drop down on the ground, but not entierly, so we have some free enegry from this see-saw (weight-lifting machine)
Here is illustrations of what told above:

Do I understand perpetual motion right?

Then Feyman says:

"We imagine that there are two classes of machines, those that are not reversible, which includes all real machines, and those that are reversible, which of course are actually not attainable no matter how careful we may be in our design of bearings, levers, etc. We suppose, however, that there is such a thing—a reversible machine—which lowers one unit of weight (a pound or any other unit) by one unit of distance, and at the same time lifts a three-unit weight. Call this reversible machine, Machine A. Suppose this particular reversible machine lifts the three-unit weight a distance X. Then suppose we have another machine, Machine B, which is not necessarily reversible, which also lowers a unit weight a unit distance, but which lifts three units a distance Y. We can now prove that Y is not higher than X; that is, it is impossible to build a machine that will lift a weight any higher than it will be lifted by a reversible machine. Let us see why. Let us suppose that Y were higher than X. We take a one-unit weight and lower it one unit height with Machine B, and that lifts the three-unit weight up a distance Y. Then we could lower the weight from Y to X, obtaining free power, and use the reversible Machine A, running backwards, to lower the three-unit weight a distance X and lift the one-unit weight by one unit height. This will put the one-unit weight back where it was before, and leave both machines ready to be used again! We would therefore have perpetual motion if Y were higher than X, which we assumed was impossible. With those assumptions, we thus deduce that Y is not higher than X, so that of all machines that can be designed, the reversible machine is the best. "

1. Reversible machine is the machine that can lift one weight by lowering another and returns to it's inital state, without any external source of enegry, so it can lift weight again?
2. How Machine B can obtain free power by lowering weight from Y to X so it can work again?
I imagine myself that we need to lower weight by ourselfs (that means putting power to lower three-unit weight) so when we release the machine, it's will bring the three-unit weight (and machine) to it's balance state (Height = Y). So where is free enegry and how does it explain that Machine B becomes perpetual motion machine?

I would be glad to get any help.
Also English is not my native language, so I'm sorry, if I've been wrong somewhere

 

[Andrew Mason]

1. Reversible machine is the machine that can lift one weight by lowering another and returns to it's inital state, without any external source of enegry, so it can lift weight again?
2. How Machine B can obtain free power by lowering weight from Y to X so it can work again?
I imagine myself that we need to lower weight by ourselfs (that means putting power to lower three-unit weight) so when we release the machine, it's will bring the three-unit weight (and machine) to it's balance state (Height = Y). So where is free enegry and how does it explain that Machine B becomes perpetual motion machine?

I would be glad to get any help.
Also English is not my native language, so I'm sorry, if I've been wrong somewhere

This is not Feynman at his most illuminating. There are different ways to show that Y > X leads to free energy, which contradicts our premise that energy cannot be created.

Here is another way to look at it:

The reversible machine A raises the 3 unit weight a vertical distance X by lowering the unit weight by a distance 3X. A is then reversed and lowers the 3 unit weight a vertical distance X (from height X to height 0) and raises the unit weight by a distance 3X. This is the starting position.

Assume that machine B then raises the 3 unit weight a height Y > X by lowering the unit weight a distance 3X (i.e from 3X to 0). Then we run machine B backward by lowering the 3 unit weight by a distance Y-X, which raises the unit weight to a height above 0, call it δ. We then run the reversible machine A to lower the 3 unit weight the rest of the way (from height X to height 0) and this raises the unit weight a distance 3X. So the unit weight is now at a height 3X + δ and the 3 unit weights are at 0, the same position as when we started. So there has been a net gain of height δ by the unit weight. This contradicts the premise. So it is not possible that Y > X.

AM

 

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We then run the reversible machine A to lower the 3 unit weight the rest of the way (from height X to height 0) and this raises the unit weight a distance 3X. So the unit weight is now at a height 3X + δ and the 3 unit weights are at 0, the same position as when we started.

I'm confused. Maybe you're talking here about Machine B instead of A? Where we can get 3X + δ with Machine A? The key part for my understanding here is how exactly Machine B work backwords? If I image myself simple pulley with two weights we can't run it backwards, it only can lower one weight and raise another, right? Even if pretend that we have complicated system of pulleys that work forward and backwards (Machine B): So we lower 1 unit weight a distance 3X and raise 3 unit weight on height Y, then run it backwards and 3 unit weight lowers on X and 1 unit weight raises on height δ then we finally lower 3 unit weight a distance X and see that our 1 unit weight raises a 3X distance, so now it's on 3X + δ, instead of it raising on 3X - δ?

 

[Andrew Mason]

Don't get hung up on how the machines work. Machine B raises the 3 unit weights a height Y>X. When it then lowers the 3 units Y-X it generates some non-zero amount of energy. Let's say it does not raise the one-unit weight at all - it creates heat. When you then use machine A to lower the 3 unit weights and the one unit weight to their original position you end up back where you started except that you have generated some heat. If you keep doing that you keep generating more heat but you always end up in the some position you started with: that means you are creating free energy.

AM

 

[Est120]

Then we run machine B backward by lowering the 3 unit weight by a distance Y-X

B is not necessarily reversible, "running B backward" needs some additional external work to be done, just A (reversible machine) does this without any external influence, just all by itslef

 

[Andrew Mason]

You don't have to use machine B to lower the 3 unit weight by the distance Y to X. You can lower it any way you want. Since the 3 unit weight (mass = 3u) at Y has 3ug(Y-X) more potential energy than at X it can use that change in potential energy to run a light bulb or charge a battery or generate heat.

To recap:
1. The energy input for both Machine A and Machine B is the lowering of the unit mass by one distance unit.
2. The work done by each machine using that same energy input is raising the 3u mass. In A's case, it is raised a distance X. In B's case it is raised a distance Y.
3. Since A is reversible, it can keep repeating the cycle (lifting 3u a height X and lowering the one unit mass a unit of distance and reversing this by lowering the 3u and raising the one unit mass to their original starting positions) without adding energy or losing energy to the surroundings.
4. We want to show that if Y > X then B creates energy.
5. We use the energy generated when lowering of the 3u by the height difference Y-X to charge an initially dead battery, say, but we do not change the position of the one-unit mass.
6. We disconnect machine B and use machine A to then lower the 3u and raise the one unit mass to their respective original starting positions.
7. We repeat, each time charging the battery a bit more until the battery is fully charged.

The result, if Y>X, is that you have a system that charges an initially dead battery but ends up in the same initial state (ie. with no energy added), thereby creating a perpetual motion machine of the first kind.

aM