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I am having some trouble with the following passage in section 4.2 of vol. I of the Feynman Lectures on Physics:

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.

Figure 4.1

Figure 4.1

The highlighted sentence feels superfluous. I understand that he wanted to avoid using specific units of measurement, but why is the machine presented as one that only lifts three units when just a couple of sentences later, the same machine is said to lift one unit and lower the three units? Why not say "this machine is a see-saw"?

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  • $\begingroup$ "FLCH42" is not an appropriate abbreviation. Spell it out in full. You have plenty of space in the body, and there is no need to add the reference in the title. $\endgroup$
    – Emilio Pisanty
    Commented Nov 24, 2020 at 14:28
  • $\begingroup$ Thanks for your feedback, @EmilioPisanty. My intention was to create a keyword that would better link all questions about Feynman Lectures Chapter 4.2, but you're right, I should use the full name just like Gary P did here. $\endgroup$
    – Paul Razvan Berg
    Commented Nov 24, 2020 at 15:17
  • $\begingroup$ Trying to tie together questions at that level of granularity (specific section of a specific chapter of a specific book) is pretty pointless. If you want a way to tie together questions regarding the Lectures as a whole, it already exists. Most importantly, though: the title has much more important things to do (read: actually explaining what the question is about) than to waste space on references. $\endgroup$
    – Emilio Pisanty
    Commented Nov 24, 2020 at 17:31
  • $\begingroup$ Appreciate your feedback :) $\endgroup$
    – Paul Razvan Berg
    Commented Nov 24, 2020 at 20:21

2 Answers 2

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In practice, it is a much more common situation that you would want to use a small amount of force (and a long lever arm) to produce a very large force that can lift a very heavy mass, than vice versa.

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The machine is reversible - it can either raise three units of weight while lowering one unit of weight or it can raise one unit of weight while lowering three units of weight. Either way round, we don’t have to put any work into the machine to get it to operate (it is tempting to say it requires no input of energy, but Feynman is avoiding using the word energy at this point in his explanation). We just have to give the machine a small nudge to start it off in one direction or the other - this nudge is the “little weight” (small amount of weight) that is added or removed from one pan or the other.

Note the although the diagram shows a beam balance (also called a see-saw or teeter-totter) the reversible machine does not have to take this form - it could instead be a hydraulic cylinder or a system of frictionless pulleys etc.

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  • $\begingroup$ You got to the crux of my confusion: if the machine can either raise three units of weight while lowering one unit of weight or it can raise one unit of weight while lowering three units of weight, why start the explanation with "this machine lifts three units strong"? $\endgroup$
    – Paul Razvan Berg
    Commented Nov 24, 2020 at 11:43

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