Why a non-reversible weight lifting machine cannot lift higher that a reversible one is related to my current post, but everyone in that thread seems to have understood how Feynman's simple machine works visually. I haven't.
And the excerpt from the Feynman Lectures on Physics Chapter 4.2:
A very simple weight-lifting is shown in Fig. 4-1. This machine lifts 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.
Feynman is blending terms, which makes visualisation hard. The said machine seems to be a see-saw, that I understand. But I can't follow whether the four squares placed on the balance pans are the "units" or the "weights". Here's why:
- We are told that there are "three units" placed on one balance pan. From this we should conclude that one square = one unit, because there are three squares on the left-hand balance pan.
- But we are also told that this machine lifts "three units strong", from which we should conclude that one square = one weight, because the single square on the right-hand balance pan is the one that is lifted. If this is true, it should follow the figure is poorly designed. Squares with higher unit "strongs" should either be drawn bigger or annotated with numbers that flag their "strongness".
I tend to lean on explanation #2, since it could account for the "three units placed on one balance pan" part, that is, the single square on the right-hand balance pan is weighing three units "strong".
To make matters worse, he is using the verb "lift" with two different meanings. The first is getting the squares in an upper position relative to the ground, while the second is about getting squares off the machine (when "lift" is used in conjunction with "off").
What are the squares in the figure? Units or weights? What does he mean by "in order to get it actually to work"?