Newton's Law of Conservation- Based on a Fallacy?

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Main Question or Discussion Point

Do you ever lie awake at night worrying about natural laws?

Conservation Laws

If a system does not interact with its environment in any way, then certain mechanical properties of the system cannot change. They are sometimes called "constants of the motion". These quantities are said to be "conserved" and the conservation laws which result can be considered to be the most fundamental principles of mechanics. In mechanics, examples of conserved quantities are energy, momentum, and angular momentum. The conservation laws are exact for an isolated system.

http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html
Isolated Systems

An isolated system implies a collection of matter which does not interact with the rest of the universe at all - and as far as we know there are really no such systems. There is no shield against gravity, and the electromagnetic force is infinite in range. But in order to focus on basic principles, it is useful to postulate such a system to clarify the nature of physical laws. In particular, the conservation laws can be presumed to be exact when referring to an isolated system:

Conservation of Energy: the total energy of the system is constant.
Conservation of Momentum: the mass times the velocity of the center of mass is constant.
Conservation of Angular Momentum: The total angular momentum of the system is constant.
Newton's Third Law: No net force can be generated within the system since all internal forces occur in opposing pairs. The acceleration of the center of mass is zero.

http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#isosys
How can we 'focus on basic principles', or indeed 'clarify the nature of physical laws' by using a model that doesn't exist in nature?
 
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Answers and Replies

  • #2
phinds
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I don't really understand what your problem is. Have you never heard of thought experiments? What would be the difference?
 
  • #3
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I don't really understand what your problem is. Have you never heard of thought experiments? What would be the difference?
The problem is there are no isolated systems.

If a system does not interact with its environment in any way, then certain mechanical properties of the system cannot change.
All systems interact with their environments, so this is an utterly meaningless thing to say. In other words, the mechanical properties of all systems can change.

They are sometimes called "constants of the motion".
...which don't exist.

These quantities are said to be "conserved" and the conservation laws which result can be considered to be the most fundamental principles of mechanics.
Meaning the 'most fundamental principles of mechanics' are baseless.

Substitute 'isolated systems' for 'translucent purple elephants' and you get an idea of how absurd all this is.

In mechanics, examples of conserved quantities are energy, momentum, and angular momentum. The conservation laws are exact for an isolated system.
How would anyone know this unless they'd studied the interaction of energy and momentum in a genuinely isolated system?

Claiming a system is self-enclosed enough to be 'almost isolated', and considering this as good enough, is no less ridiculous than artificially inducing morning sickness in a barren woman and then declaring she's 'almost pregnant' as a way to study fetal development in the womb.
 
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  • #4
Pengwuino
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Substitute 'isolated systems' for 'translucent purple elephants' and you get an idea of how absurd all this is.

How would anyone know this unless they'd studied the interaction of energy and momentum in a genuinely isolated system?

Claiming a system is self-enclosed enough to be 'almost isolated', and considering this as good enough, is no less ridiculous than artificially inducing morning sickness in a barren woman and claiming she's 'almost pregnant' as a way to study fetal development in the womb.
Except the entire world around you has been studied, millions of experiments run, consistent results shown, and a tremendous understanding of the universe made despite the fact that we know there is no such thing as isolated systems. The problem with purple elephants is that assuming purple elephants doesn't give you anything consistent with the world. Assuming an electromagnetic wave traveling through a vacuum as the basis of an experiment, however, does give you consistent, usable results.

That's like saying how dare we use mathematics to build this society when all of mathematics is a human construct
 
  • #5
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Of course all these mathematical things don't exist in nature!! Of course there is no such thing as constant of motion and spacetime. But that isn't the point.

The point is that the mathematics forms an APPROXIMATION of nature. That's all physics claims to be: an approximation. Physics is useful in predicting nature. And it's darn good at it too!! Newton's laws have been tested over and over again and are shown to hold. So Newton's Laws isn't a fallacy since it has been confirmed by EXPERIMENT.
 
  • #6
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How can we 'focus on basic principles', or indeed 'clarify the nature of physical laws' by using a model that doesn't exist in nature?
You seem to be equating "approximation" or "limiting case" with "fallacy". I don't think that association is correct.
 
  • #7
DaveC426913
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A perfect circle does not exist in nature either. Does that mean it is meaningless to use them in our equations?

octagon, the tenet of science is to separate confounding factors from relevant causes and effects.

The fact that I can't keep all extraneous heat out of my black box that's capturing sunlight does not mean it is meaningless to study exactly how much energy the light does contribute to the box.

We pull the factors apart, learn how they work out of context, then, when we apply our knowledge to the real world, we factor in those those other effects. That's when science research turns into engineering.
 
  • #8
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Thanks for the replies.

How do you see this relating to theoretical physics?
 
  • #9
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How do you see this relating to theoretical physics?
What do you mean?? Theoretical physics is working fine...
 
  • #10
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octagon said:
...a model that doesn't exist in nature...
How do you see this relating to theoretical physics?
Probably it can help you solve your problems considering the universe as "the isolated system" that exists in nature, all other systems are just approximations, idealizations. Theoretical physics deals with idealizations, ideal models are verified by "real-world"-experiments in physics.
 
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  • #11
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What do you mean?? Theoretical physics is working fine...
Well, the 'arrow of time' for example.

That's based on the idea of entropy increasing in an isolated system, right?
 
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  • #12
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There are several equivalent ways to express the second law of thermodynamics, and many of them do not refer to isolated systems. However, it is usually easiest for students to understand the implication of the law on an isolated system. This is a pedagogical technique, not an essential part of the law.

Personally, I think that the idea of a "system" is much more unnatural than the idea of an "isolated" system. I mean, a system is a purely mental and human construct, you simply arbitrarily pick some line and say that stuff on one side is the system and stuff on the other side is not. The line doesn't have to correspond to anything physical and it can move or whatever. Once you have swallowed the idea of a "system" being something real that you can write physical laws about then the idea of writing laws about "isolated" systems seems trivial.
 
  • #13
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OK, I think I understand now.

Again, thanks all, for taking the time to respond.
 
  • #14
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How can we 'focus on basic principles', or indeed 'clarify the nature of physical laws' by using a model that doesn't exist in nature?
If you learn what science is and does, you will discover that a scientific model is not the same than the reality that it represents.
 
  • #15
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Well, the 'arrow of time' for example.

That's based on the idea of entropy increasing in an isolated system, right?
No. The second law is a statement about the production of entropy, so that it applies to closed, isolated, or open systems. Indeed, the thermodynamics of open systems has worked very well during last two centuries and at leats two guys won a Nobel prize for their work in this field.

I know this a bit. Precisely, I have just finished this Friday a paper on a new definition of heat, for open systems, that extends the usual definitions used by DeGroot, Mazur, Callen, Prigogine, Haase, and others in the thermodynamics of irreversible processes.
 

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