# Does entropy apply to Newton's First Law or does “acted upon” always require an external factor?

First law: Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant speed in a straight line.

Doesn't the law of increasing entropy affect all objects though, since they are all in the closed system of the universe at large, and therefore they are all subject to slowing down, regardless of the containing medium, given enough time?

I guess what I'm curious is, can there ever be a body that will remain at uniform motion or uniform rest given that entropy must increase?

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## 2 Answers

Yes, it will (in the classical picture) continue forever, even with its entropy increasing, the entropy increase just means that some of the potential energy within the body will turn into heat (=kinetic energy), however the center of mass motion is unaffected. Entropy increase says nothing about slowing down, rather the opposite.

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@kakemonsteret, "the entropy increase just means that some of the potential energy within the body will turn into heat (=kinetic energy)" ??? totally wrong! – user1355 Feb 1 '11 at 16:27
@sb1 WHAT is wrong. – Holowitz Feb 1 '11 at 16:28

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Yes it does, "Entropy is a thermodynamic property that is a measure of the energy not available for useful work in a thermodynamic process," -wik – Holowitz Feb 1 '11 at 16:31

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@sb1 No, you are confused, ideal gases dont exist – Holowitz Feb 1 '11 at 16:43

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How exactly do you want to increase the entropy of anything, without converting potential energy into kinetic ? That is impossible – Holowitz Feb 1 '11 at 16:52
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If you ignore the microscopic explanation of entropy, entropy is just an internal state of a system, on par with the system's volume, or the number of particles in the system. If you have a gas with a fixed entropy (let's store it in a vacuum tube so it doesn't escape, and let's give the tube infinite insulation so it doesn't leech out any heat) out in deep space, and you throw it, it will just happily trail off in a straight line forever with a constant entropy, volume, and number of particles.

You get changes in entropy and whatnot only when the system in question interacts either with another system or its environment. But it really is best thinking of these things, at least on a macroscopic scale, as internal degrees of freedom of the system.

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??? Entropy of a closed system always increases – Holowitz Feb 1 '11 at 15:58

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@kakemonsteret: No: the entropy of a closed system must always either increase or remain constant. The Second Law reads $\Delta S \geq 0$, not $\Delta S > 0$. In particular, a system in equilibrium maintains constant entropy. – Jerry Schirmer Feb 1 '11 at 16:11

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barring fluctuations... – Marek Feb 1 '11 at 18:34
@Marek: sure, but at that point, the second law isn't being obeyed anyway. And which is also why I say to ignore the microscopic explanation of entropy. – Jerry Schirmer Feb 1 '11 at 21:36
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