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Here is a paragraph from Feynman lecture about conservation of energy:

Consider weight-lifting machines—machines which have the property that they lift one weight by lowering another. Let us also make a hypothesis: that there is no such thing as perpetual motion with these weight-lifting machines. (In fact, that there is no perpetual motion at all is a general statement of the law of conservation of energy.) We must be careful to define perpetual motion. First, let us do it for weight-lifting machines. If, when we have lifted and lowered a lot of weights and restored the machine to the original condition, we find that the net result is to have lifted a weight, then we have a perpetual motion machine because we can use that lifted weight to run something else. That is, provided the machine which lifted the weight is brought back to its exact original condition, and furthermore that it is completely self-contained—that it has not received the energy to lift that weight from some external source—like Bruce’s blocks.

I do not understand his definition of perpetual motion. On the one hand, he says that "If, when we have lifted and lowered a lot of weights and restored the machine to the original condition, we find that the net result is to have lifted a weight, then we have a perpetual motion machine", so we lift and lower the weights, which means the machine received energy from the outside.

On the other hand, he says that this is true provided that the machine has not received the energy from an external source.

So what does he mean?

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He's using the term "we" colloquially; he doesn't mean someone is physically running the machine and putting energy into it, but simply that the machine is allowed to run its course and complete a cycle. Perhaps you might prefer the phrasing "If, once we have allowed the machine to lift and lower a lot of weights and come back to its original condition, we find that the net result is to have lifted a weight, then we have a perpetual motion machine."

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  • $\begingroup$ Thanks. I do not understand something. When the machine lowers a weight on one side, it MUST lift the one on the other side. So what does the question "IF... the net result is to have lifted a weight..." mean? $\endgroup$ – Sipo Feb 23 '17 at 7:56
  • $\begingroup$ The point is that you're interested in the NET result - so for instance, lowering a ten-kilogram mass and raising a nine-kilogram mass (by the same about) counts as a net lowering of weights, since more weight was lowered than was raised. Feynman is referring to something like the opposite case: lower 9 kg and raise 10 kg. (More precisely, if the total potential energy of the weights increases, it counts as a net increase of weight. This phrasing makes it obvious it's impossible, since that would violate conservation of energy :P ) $\endgroup$ – Sebastian Feb 24 '17 at 16:38
  • $\begingroup$ Hey! Follow-up: what does he mean when he says "a lot of weights"? $\endgroup$ – Sipo May 5 '17 at 9:22
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    $\begingroup$ It's still colloquial; when he says "lifted and lowered a lot of weights and restored the machine to the original condition", he means that the machine has run a complete cycle (however simple or complicated it may be). $\endgroup$ – Sebastian May 5 '17 at 21:21
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    $\begingroup$ Yes, that's essentially the physical content of what he's saying. $\endgroup$ – Sebastian May 7 '17 at 19:01
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Note: Hypothetically, if it were possible, perpetual motion would be defined as motion from rest at every point at no net loss of altitude, without using depletable energy.

If those criteria can be met in a simple device, e.g. without any radioactivity, and it is possible in a closed system (ignoring metaphysics), then it is perpetual motion.

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  • $\begingroup$ Radioactivity is depletable energy, so you don't need a special exception for radioactivity. $\endgroup$ – immibis Mar 6 at 2:43

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