Science desk
< May 26	<< Apr \| May \| Jun >>	May 28 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

May 27[edit]

Feynman Lectures. Exercises PDF. Exercises 4-1...4-16[edit]

I have a general question. In exercises for Lecture 4 a principle "if system is in balance it is reversible machine" is used. But why is it true?
If a reversible machine is balanced it doesn't mean that anything else balanced is a reversible machine.
If Feynman uses this argument, then why doesn't he say that frankly in Lecture 4 (He says only "If we say it is just balanced, it is reversible and so can move up and down")?
But even with this argument there is a problem: at the moment of Lecture 4 we do not know that balanced system is a system without acceleration, because we do not know what acceleration is yet. Therefore, we can't know what a "balanced system" means. So we can't say is the reversible machine in Fig. 4–1 balanced or not at each moment of its operation.

We have:
1) Statement "balanced system is reversible machine" is unproven.
2) Statement "reversible machine is balanced system" is unproven.
3) Statement "if all reversible machines are balanced then all balanced systems are reversible machines" lacks logic. Username160611000000 (talk) 08:10, 27 May 2017 (UTC)[reply]

That part isn't quite as clear as the rest, but it does make sense. If you have a "machine" in the classical sense, specifically an inclined plane, with weights on the incline and vertically, then if one end pulls down i.e. it is unbalanced, it is not reversible. Presumably this is in the sense that once the one weight hits the ground, its energy is dissipated and (by conservation of energy) there's no way to reset the whole thing how it was without putting some energy back in. He is taking conservation of energy on blind faith, to be sure. Now, if you have the same machine and it is precisely balanced, then you can push it either way as you like, because, well, I think that's what balanced means. But ... yes, balanced is an imprecise term. After all, a screwdriver can be balanced on its tip, and in theory, you could have it sitting like that and with a push send it one way or the other, yet that is not reversible. The key distinction there though is that the screwdriver doesn't stay balanced. The inclined plane example, in theory, you can give it a little teeny weeny itsy bitsy push, and if it's a perfectly reversible machine, watch it creep down or up, one way or the other, over months and years and whole precessions of the earth, and yet, if you give it a little more push, or push it a little back the same way, it will then go at that new rate the same way. And so you see there that because it remains continually balanced, it never is in the point of giving up its energy, i.e. it does no mechanical work on the weights to get them moving this way or that, beyond what you yourself have put in. Wnt (talk) 20:12, 27 May 2017 (UTC)[reply]

@Wnt: He is taking conservation of energy on blind faith -- No. He takes the impossibility of the perpetual motion (in the sense of continual supply of extra energy) on blind faith (later in the Lect.4 he says that experiment confirms the impossibility, so it's not a blind faith). And from that he derives the conservation of energy law (for reversible machine and for gravitation potential energy only). Reversible machine is a machine that can lift 1 unit weight 1 meter by lowering N units weight X meters, and can lift N units X meters by lowering 1 unit 1 meter.
Username160611000000 (talk • contribs) 12:22, 30 May 2017 (UTC)[reply]

I'm pretty sure he's using the term "balance" here to mean what is often termed "equilibrium" in the physical sciences. Chemical equilibrium, for example, is closely tied to the concept of the reversible reaction. In general, a system that is at an equilibrium resists changes in its status by returning to equilibrium (see Le Chatelier's principle for the chemical law), and by definition is reversible. That is, if something is in an equilibrium state "A" and some action puts it in state "B", then it will spontaneously return to state A. That's a reversible system. With the screwdriver example, if you balance the screw driver on its tip, it is NOT in equilibrium, because a small changes causes it to leave its initial state, not return to it. By contrast, a screwdriver lying on its side IS in equilibrium, because a small change causes it to return to its initial state. By definition, all equilibrium systems are reversible. The opposite statement is NOT necessarily true, however, not all reversible systems are in equilibrium; in the screw driver example BOTH processes are reversible, but only one is in equilibrium. --Jayron 32 12:51, 30 May 2017 (UTC)[reply]

@Jayron32: That is, if something is in an equilibrium state "A" and some action puts it in state "B", then it will spontaneously return to state A. I don't understand. According to Feynman a reversible machine is a machine capable of equally willingly passing from state A to state B and from state B to state A:

...
A very simple weight-lifting machine is shown in Fig. 4–1. This machine lifts weights 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. This we disregard, temporarily. Ideal machines, although they do not exist, do not require anything extra. A machine that we actually use can be, in a sense, almost reversible: that is, if it will lift the weight of three by lowering a weight of one, then it will also lift nearly the weight of one the same amount by lowering the weight of three.

— Feynman • Leighton • Sands, The Feynman Lectures on Physics, Volume IAnd with the screwdriver, it's not a machine at all. It seems it's like Unstable equilibria from this article [1], but I do not think that this has something to do with lecture 4. Username160611000000 (talk) 16:58, 30 May 2017 (UTC)[reply]

It sort of depends on how you set up your systems. For example, in the case of chemical equilibria, a system is only at chemical equilibrium if its temperature is held constant; the equilibrium conditions (related to the equilibrium constant) is tied to a specific temperature. If you let temperature vary freely, you get a truly reversible chemical processes. In Feynman's case, he's established a reversible system because it can be run in both directions with small inputs of energy; and it's balanced (equilibrium) because small changes in any direction will self-correct. His example may be simplified by imagining a lever with the fulcrum offset to one side, such that there is, say, one meter of lever on side A and three meters of lever on side B. If we THEN put 3 kilograms of weight on side A and 1 kilogram of weight on side B, we have our simple, balanced (equilibrium) machine. We can do work in either direction with similar effort (that is, I can push down either side, and get the opposite side to lift up). That's a simple machine which is also a reversible machine. It's also a balanced machine (more properly it is at "equilibrium") because it is self correcting; if I nudge the lever and let it go, it returns to level on its own. --Jayron 32 04:40, 31 May 2017 (UTC)[reply]

if I nudge the lever and let it go, it returns to level on its own. Why? Actually (what we do not yet know at the moment of Lect.4) to accelerate the machine to some speed we must apply a force. And when the machine gains speed, we turn off the force, then the machine continues movement forever . Username160611000000 (talk) 12:46, 31 May 2017 (UTC)[reply]

I think I understand. Reversible machine is a machine for which a very small push (or as Feynman says "extra") brings it to movement (0.5 cycle). So before that push the machine is idle (v=0). If machine is stationary it is balanced, because one of the criteria of "balanced" is fact v=0 (second is v=const). Username160611000000 (talk) 16:53, 31 May 2017 (UTC)[reply]
All info about thermodynamics, temperature, screwdrivers, chemical equilibria is unknown (at Lect.4) , off-topic and complicated. Username160611000000 (talk) 17:00, 31 May 2017 (UTC)[reply]

Typical revolutions per minute of a fidget spinner[edit]

Not a specialised one designed to go ridiculously fast. Just a normal, hand-spun one. What sort of ballpark are we talking? 10rpm? 1000rpm? Amisom (talk) 21:05, 27 May 2017 (UTC)[reply]

This is still a very new field of research so there won't yet be much published literature on this particular aspect.----Seans Potato Business 21:57, 27 May 2017 (UTC)[reply]

A Fidget spinner is a type of stress-relieving toy. A video of children playing with the toy shows it spins at a rate a child can easily start and stop, apparently similar to a Gramophone record i.e. 16 to 78 r.p.m.. Blooteuth (talk) 22:55, 27 May 2017 (UTC)[reply]

A fidget spinner is hand spun to about 1000rpm in this YouTube video (1:06 minutes in). --Modocc (talk) 23:01, 27 May 2017 (UTC)[reply]

About 20,000 rpm or more to get failure. Count Iblis (talk) 23:54, 27 May 2017 (UTC)[reply]

Is there any pattern to warning symbols?[edit]

I see sometimes common warning symbols in orange squares or white diamonds with red outlines or yellow triangles with black outlines or white triangles with red outlines. Is there some situation where one is preferred? Can an nuclear trefoil, for example, be put in any of these?

See the article about [Hazard symbol]s. Their use of hazard symbols is often regulated by law and standards organisations who direct different colors, backgrounds, and borders. The radiation Trefoil symbol was originally majenta on a yellow or blue background but is now internationally recognized drawn in black, see Hazard symbol#Ionizing radiation trefoil warning symbol. See the article Warning sign about national traffic hazard signs. Blooteuth (talk) 22:44, 27 May 2017 (UTC)[reply]

See also Globally Harmonized System of Classification and Labelling of Chemicals and GHS hazard pictograms. Also Hazchem used in some Commonwealth countries. Alansplodge (talk) 23:18, 27 May 2017 (UTC)[reply]