A question with regard to Hooke's and Kepler's laws

[Florian Geyer]

TL;DR Summary

Shall we consider the previous "laws" as physical laws? why?

Hello esteemed members,
I have been studying some physics and I have the mentioned laws, but I have understood that the first law (elasticity modulus equals the stress over strain) $e=\frac{\sigma}{\epsilon}$ is valid only to limited range, and the second one is just a result of Newton's second law.
Thus I concluded that they shall not be considered as laws since laws must be general and not resulted from another law, and they must have adequately large range of validity, which I do not think is the case of any of these laws.

Thanks in advance for reading/considering replying.

 

[berkeman]

I have been studying some physics and I have the mentioned laws, but I have understood that the first law (elasticity modulus equals the stress over strain) $e=\frac{\sigma}{\epsilon}$ is valid only to limited range,

Of course. What happens when you stress a material beyond it's elastic limit (beyond the Yield Strength)?

https://en.wikipedia.org/wiki/Stress–strain_curve

 

[Hill]

[Kepler's laws are] just a result of Newton's second law.

No, Kepler's laws cannot be derived just from Newton's second law.

 

[Vanadium 50]

OP. what do you think a "law" is? It is certainly not something always true - Ohm's law describes ideal resistors, but not everything is an ideal resistor. Or even a resistor.

 

[Dale]

TL;DR Summary: Shall we consider the previous "laws" as physical laws? why?

laws must be general and not resulted from another law, and they must have adequately large range of validity

I have never heard those requirements.

Most laws are just called laws because historically they have been called laws. I guess you could make some rigid definition and exclude Hooke’s law under that definition, but people will still call it Hooke’s law.

 

[topsquark]

TL;DR Summary: Shall we consider the previous "laws" as physical laws? why?

Hello esteemed members,
I have been studying some physics and I have the mentioned laws, but I have understood that the first law (elasticity modulus equals the stress over strain) $e=\frac{\sigma}{\epsilon}$ is valid only to limited range, and the second one is just a result of Newton's second law.
Thus I concluded that they shall not be considered as laws since laws must be general and not resulted from another law, and they must have adequately large range of validity, which I do not think is the case of any of these laws.

Thanks in advance for reading/considering replying.

Personally, I really don't care what they are called... "laws" works just fine for me. We have ro refer to them by some means, however. In their ranges of validity I see no reason not to call them laws. And not all of the principles that Physics is based on are called laws anyway. Some are "theorems" and others are just names of equations or concepts that we use.

From your statement, I guess you don't feel that we have any Laws in Physics? I mean, not even Newton's 2nd Law always holds. You seem to be reaching for some kind of Axiomatic development of Physics, I take it? So far we don't yet know enough to make a list of the most basic "laws" that govern the Universe, so the list of laws could be constantly changing as we learn new things. Again, I really don't care what they are called, in the end. But we have to call them something to give them some kind of Historical label so we can refer to them by a consistant name for more than a decade or so. Just out of curiosity, what would you prefer us to call them by?

-Dan

 

[Florian Geyer]

No, Kepler's laws cannot be derived just from Newton's second law.

This is part of freshman curriculum, cannot we consider it as a derivation of Kepler's law?

 

[Hill]

cannot we consider it as a derivation of Kepler's law?

Yes, it is a derivation of the Kepler's law, but as you see, it is not derived just from the Newton's second law but requires applying the gravitation law.

 

[Florian Geyer]

OP. what do you think a "law" is? It is certainly not something alwasy true - Ohm's law describes ideal resistors, but not everything is an ideal resistor. Or even a resistor.

mmm well, I think of a law as something that has a wide range of validity, at least like Newton's laws, yes they are not valid at the quantum or astrophysical realms, but they still can be considered true to the whole range of classical mechanics.
I still struggle to grasp the differences between laws, theories, equations, relations... I mean if it is a matter of arbitrary names then why shall we bother calling some of them laws, and others theories?
After the previous replies I think the only thing I seem to understand well is principles, which as far as I know always true no matter what.

 

[Florian Geyer]

Yes, it is a derivation of the Kepler's law, but as you see, it is not derived just from the Newton's second law but requires applying the gravitation law.

Ok, but since we have derived it from them, is not it better to call them laws, and to call it a relation? or results?
Well I know Kepler had done his "law" before Newton, but after Newton made his laws I think it would be more suitable to call them results, relations or anything else!

 

[Hill]

since we have derived it from them, is not it better to call them laws, and to call it a relation? or results?

I don't think so. IMO, they are results, relations, and laws. I don't see why one is better than another.

 

[Florian Geyer]

I have never heard those requirements.

Most laws are just called laws because historically they have been called laws. I guess you could make some rigid definition and exclude Hooke’s law under that definition, but people will still call it Hooke’s law.

Yes yes, I may have not understood the meaning of a law well, after the discussion with the members here I came to the same conclusion.
However regarding

Most laws are just called laws because historically they have been called laws.

Can you suggest to me a resource for this? I will be very thankful to you.

 

[Florian Geyer]

I don't think so. IMO, they are results, relations, and laws. I don't see why one is better than another.

Do you mean they are arbitrary names? just something like this?

 

[Hill]

Do you mean they are arbitrary names? just something like this?

Yes, pretty much. Other names are "equation" (as in "Dirak equation"), "inequality" (as in "Bell's inequality"), "transformation" (as in "Lorentz transformation"), even "diagram" (as in "Feinman diagram").

 

[Florian Geyer]

Personally, I really don't care what they are called... "laws" works just fine for me. We have ro refer to them by some means, however. In their ranges of validity I see no reason not to call them laws. And not all of the principles that Physics is based on are called laws anyway. Some are "theorems" and others are just names of equations or concepts that we use.

From your statement, I guess you don't feel that we have any Laws in Physics? I mean, not even Newton's 2nd Law always holds. You seem to be reaching for some kind of Axiomatic development of Physics, I take it? So far we don't yet know enough to make a list of the most basic "laws" that govern the Universe, so the list of laws could be constantly changing as we learn new things. Again, I really don't care what they are called, in the end. But we have to call them something to give them some kind of Historical label so we can refer to them by a consistant name for more than a decade or so. Just out of curiosity, what would you prefer us to call them by?

-Dan

Aha I understand your point of view, well how about principles? they are always valid as far as I know, and when a theory contradicts a principle then we modify the theory, I think this is how neutrinos have discovered.
Now I came to the conclusion that the title law we attribute to a fact is arbitrary and connected to how physicists have perceived them at first.

So far we don't yet know enough to make a list of the most basic "laws" that govern the Universe, so the list of laws could be constantly changing as we learn new things.

Do you extend this opinion even to principles?

 

[Florian Geyer]

Yes, pretty much. Other names are "equation" (as in "Dirak equation"), "inequality" (as in "Bell's inequality"), "transformation" (as in "Lorentz transformation"), even "diagram" (as in "Feinman diagram").

Some last things to conclude:
1- I think we need to use some resources, is there a comprehensive dictionary for physics concepts?
2- Do think these names are "arbitrary" even when we are talking about physical principles?

 

[PeroK]

Do you mean they are arbitrary names? just something like this?

It might be an interesting exercise for a student of the history of physics to look at the different "laws" and find a plausible reason why some key things are called a "law", others a "principle" and others an "equation".

For everyone else, I suggest it's a waste of time!

 

[Hill]

Some last things to conclude:
1- I think we need to use some resources, is there a comprehensive dictionary for physics concepts?
2- Do think these names are "arbitrary" even when we are talking about physical principles?

My answer would be the same:

I suggest it's a waste of time!

 

[Dale]

Can you suggest to me a resource for this?

Sorry, I don’t know any such resource.

 

[Florian Geyer]

It might be an interesting exercise for a student of the history of physics to look at the different "laws" and find a plausible reason why some key things are called a "law", others a "principle" and others an "equation".

For everyone else, I suggest it's a waste of time!

mmm well, when I learn about any new thing I determine the depth to which I will go... in this case I think it would be a good idea to have a look on something simplified to a length of something like 10 pages or less.
I am not sure if this also is a waste of time, I look forward to hear from you.

I want to extend this a little further and ask if you also consider all the study of history or philosophy of physics as a waste of time for those who aspire to be physicists and do research.

I have heard two contradictory opinions in this regard.
1- I think Maxwell has recommended physicists to read about the history of physics. (unfortunately I do not remember where I have read about this... it is something like 5 years ago).
2- Feynman once said:

"Philosophy of science is about as useful to scientists as ornithology is to birds."

Now, is this the same as the case of history of physics?
Will not I need to study some books like kuhn's the structure of scientific revolutions, or Popper's The Logic of Scientific Discovery.

Why? why not please explain.

 

[PeroK]

I am not sure if this also is a waste of time, I look forward to hear from you.

There is nothing stopping you researching it yourself. You could rename Newton's laws to Newton's humble suggestions for the advancement of science. That doesn't change anything. Not to a scientifically-minded person.

I want to extend this a little further and ask if you also consider all the study of history or philosophy of physics as a waste of time for those who aspire to be physicists and do research.

The history of science is a fascinating subject. Whether you have time to study it while doing a degree is a different matter.

The philosophy of science to science is a contentious subject. As are Popper's views on science.

 

[Hill]

For the history and philosophy of science for physicists I'd recommend THE EVOLUTION OF PHYSICS BY ALBERT EINSTEIN & LEOPOLD INFELD.

 

[Florian Geyer]

There is nothing stopping you researching it yourself. You could rename Newton's laws to Newton's humble suggestions for the advancement of science. That doesn't change anything. Not to a scientifically-minded person.

The history of science is a fascinating subject. Whether you have time to study it while doing a degree is a different matter.

The philosophy of science to science is a contentious subject. As are Popper's views on science.

Now I understand you better.
Thank you

 

[Florian Geyer]

For the history and philosophy of science for physicists I'd recommend THE EVOLUTION OF PHYSICS BY ALBERT EINSTEIN & LEOPOLD INFELD.

Thank you a lot.
I will put it in my TBR list.

 

[Dale]

I want to extend this a little further and ask if you also consider all the study of history or philosophy of physics as a waste of time for those who aspire to be physicists and do research.

For someone who aspires to do research, IMO, the study of philosophy is a waste of time. Everything useful for a researcher, from the last century or two of philosophy (especially Popper and Occham), falls out automatically from Bayesian statistics. So spend time studying that instead and you will wind up with not just the useful concepts but also a concrete set of tools for implementing them in your research.

The history is more useful. It can give you a sense of how many dead ends and false leads are pursued. It can also give you a sense of how non linear the actual process of using the scientific method is. Most likely none of that will lead you to a new discovery, but it may help you continue past some failures.