FLCH42 stands for Feynman Lectures Chapter 4.2, known to frequently confuse budding physicists. This is the fourth in a personal series of questions I had about the chapter.
Excerpt:
Call this reversible machine, Machine A. Suppose this particular reversible machine lifts the three-unit weight a distance distance X. Then suppose we have another machine, Machine B, which is not necessarily reversible, which also lowers a unit 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 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 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 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 X, obtaining free power, and use the reversible Machine A, running backwards, to lower the three-unit weight a distance distance X and lift the one-unit weight by one unit height.
I understand reductio ad absurdum in general, so that is not the issue. The "obtaining free power" is where I got lost. How is this free power? He kind of implies that there is an external force applied to the machine when he says "we could lower the weight from Y to X".
In addition, I struggle to understand his explanation visually. When he says to use the reversible Machine A that runs "backwards", does he mean that someone picks up the three-unit weight with their hands and places it on the balance pan on Machine A?
