Hi I'm having a very hard time trying to picture an argument in Feynmans physics volume one ). I can't picture this example he is talking about, so I wrote it down from the book and really hope someone can help me answer the question in the bottom.(adsbygoogle = window.adsbygoogle || []).push({});

From the book (everything needed to know is included)

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 aLet us see why.

weight any higher than it will be lifted by a reversible machine.

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

V. 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.

I don't get this part:

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.

Why can't Y (the distance lifted by machine A) be highter than X (distance lifted by machine B)?? I assume both machines have same proportions (are able to reach the same maximum height) and both machines lift 3 units of weight a distance by lowering one unit a distance. So what physical law says that Machine B lift 3 units as high as machine A?

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# Feynman's reversible lifting machine analogy- help

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