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I was reading Feynman lectures and found the following

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. Of course, we realize that with any actual lifting machine, we must add a little extra to get it to run. This we disregard, temporarily. Ideal machines, although they do not exist, do not require anything extra. A machine that we actually use can be, in a sense, almost reversible: that is, if it will lift the weight of three by lowering a weight of one, then it will also lift nearly the weight of one the same amount by lowering the weight of three.

Now this left me thinking how a lever could end up being reversible. I am not concerned here whether or not reversible machines exist and ready to be in an idealized world, but still it is not clear to me how the machine would work reversible.

What I think is that a one unit mass would lift a three unit mass if the distance from pivot is as desired and would be dependent on net torque on the bar due to force of gravity on masses. But then, when torque is balanced out, then how could this lever operate in a reverse fashion without any outside energy source, even when situation is completely ideal.

Is it that Feynman means here that we will obviously change the distance from the pivot of the weights and then allow the machine to work reversibly ? But then I wouldn't think it to be a machine as we have altered it and its is not self contained.

I know that there is something that I am forgetting here to include and Feynman ids obviously right, but I am not able to grasp it. I want to clearly understand his reasoning here, and thus asking for help. I don't see any reason for the machine to work in an opposite fashion, even when we provide it with slifhtest force, in theory.

Any help is appreciated. Thanks.

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A reversible process would take infinitely long time. The tiny weight that Feynman postulates to set the thing in motion can, in an ideal world, be as small as you like. The smaller it is, the closer to reversible you get - and the longer one cycle takes.

In the limit of an infinitesimal weight, the process becomes reversible and takes forever.

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  • $\begingroup$ But what force will cause the direction of lever to reverse? Why would it even reverse without any external support even when we are completely ideal as required and there is no 'loss of energy'. Can you explain this part further ? $\endgroup$ – Abhinav Dhawan Feb 10 '18 at 16:20
  • $\begingroup$ Why do you think this can work without an external support? The rotation requires torque and torque requires two points where force is applied. $\endgroup$ – Floris Feb 10 '18 at 16:21
  • $\begingroup$ By external support I mean anything other than the machine and weights. $\endgroup$ – Abhinav Dhawan Feb 10 '18 at 16:22
  • $\begingroup$ What I don't understand is that even if we ignore factors like friction, how can the working of reversible machines justified ? $\endgroup$ – Abhinav Dhawan Feb 10 '18 at 16:25
  • $\begingroup$ A reversible engine is a theoretical construct. Something you can get close to, but never achieve. Like the "restated laws of thermodynamics": "1: you can't win, you can only break even; 2: you can only break even at absolute zero; 3: you can't reach absolute zero". $\endgroup$ – Floris Feb 10 '18 at 20:18

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