^indeed.

In the world that we inhabit - not to big, not too small and not too fast - newtons laws are just fine.

John Platko wrote:From Wikipedia

Second law: The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. Thus, F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object.

Regarding previous comments about relativity. Using F=mA is fine for special relativity (where A is the acceleration 4 vector) but for general relativity you would need to write: F = dP/dt. In what sense do you mean they are true? As far as we know F = dP/dt always holds. You cannot prove these in the absolute sense of mathematics. They are models of the universe based on empirical measurement.

Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. (Separately it was shown that large spherically symmetrical masses attract and are attracted as if all their mass were concentrated at their centers.) This is a general physical law derived from empirical observations by what Newton called induction.[2]

As has already been mentioned this has been super ceded by Einstein's general relativity where gravity is not treated as a force as such. For example it was known for some time that the perihelion advance of Mercury had an unexplained discrepancy which is accurately predicted by general relativity. The difference is fairly small so Newton's laws can still be used unless you want best accuracy. We still don't know for sure how gravity fits in as a quantum theory so again no absolute truth.

My question is, how true are these laws for objects like golf balls and planets? (as opposed to electrons and such) Are there exceptions, are they incomplete, are they true?

They are useful. For most Earth bound applications such as construction and general engineering, Newton's laws give suitably reliable results. I would not recommend leaping off a twenty story building to try and show Newton was wrong.

klazmon wrote:

John Platko wrote:From Wikipedia

Second law: The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. Thus, F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object.

Regarding previous comments about relativity. Using F=mA is fine for special relativity (where A is the acceleration 4 vector) but for general relativity you would need to write: F = dP/dt. In what sense do you mean they are true? As far as we know F = dP/dt always holds. You cannot prove these in the absolute sense of mathematics. They are models of the universe based on empirical measurement.

Ok. One of my questions hinged around what the model implies and whether or not the implications are true.

The classical way to determine the velocity at any time Tx is to integrate from T0, the time I let go of the ball to Tx the acceleration (F/m) of the ball. This seems to imply that the ball undergoes an infinite number of changes in velocity, with a corresponding infinite number of changes in E, from the time I let go of it until Tx. Is that true?

Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. (Separately it was shown that large spherically symmetrical masses attract and are attracted as if all their mass were concentrated at their centers.) This is a general physical law derived from empirical observations by what Newton called induction.[2]

As has already been mentioned this has been super ceded by Einstein's general relativity where gravity is not treated as a force as such. For example it was known for some time that the perihelion advance of Mercury had an unexplained discrepancy which is accurately predicted by general relativity. The difference is fairly small so Newton's laws can still be used unless you want best accuracy. We still don't know for sure how gravity fits in as a quantum theory so again no absolute truth.

My question is, how true are these laws for objects like golf balls and planets? (as opposed to electrons and such) Are there exceptions, are they incomplete, are they true?

They are useful. For most Earth bound applications such as construction and general engineering, Newton's laws give suitably reliable results. I would not recommend leaping off a twenty story building to try and show Newton was wrong.[/quote]

What is all this "truth" you speak of? What "truth" are you seeking John?

Sendraks wrote:What is all this "truth" you speak of? What "truth" are you seeking John?

There are two aspects to my question. One is more general. What does it mean when physics calls something a law? Does it mean that the law is true? In what sense is the law true? It's all very curious.

But I'm much more interested in the second aspect of my question- the scientific nuts and bolts aspect. What is actually happening when a golf ball falls. Is it falling in a continuous way? Is that the reality of what is happening? Or is some very complex series of discreet events happening which just appear to be continuous to us on our scale? What does current science say about this? Is the answer just in the realm of speculation or can an answer be supported by scientific empirical evidence that is very persuasive?

And if modern science can't shed any definitive light on whether the ball falls continuously or discreetly, can ancient science? Aristotle had some rather compelling theories about it being impossible to traverse an infinite number of state transitions. Could one apply that idea to the ball falling and conclude that it must fall in a discreet way?

John Platko wrote:
There are two aspects to my question. One is more general. What does it mean when physics calls something a law? Does it mean that the law is true? In what sense is the law true? It's all very curious.

I'm none the wiser as to what you mean by "true."

John Platko wrote:And if modern science can't shed any definitive light on whether the ball falls continuously or discreetly, can ancient science?

You're suggesting that modern science has somehow ignored earlier theories in the development of current theories?

A gap, a gap, my kingdom for a gap!

John Platko wrote:
And if modern science can't shed any definitive light on whether the ball falls continuously or discreetly, can ancient science?

No.

[/thread]

Well quite.

Where "learning" actually means "ignoring what could be learn for the sake of not understanding and attempting to drive woo shaped wedges into science."

Sendraks wrote:

John Platko wrote:
There are two aspects to my question. One is more general. What does it mean when physics calls something a law? Does it mean that the law is true? In what sense is the law true? It's all very curious.

I'm none the wiser as to what you mean by "true."

By true I mean: having all the expected or necessary qualities.

As this thread demonstrates, the two laws of newton the thread discusses are not true by the definition I plucked out of Merriam Webster. Maybe that's the answer. These two laws of Newton are not true. But that's not very satisfying because for all practical purposes for most humans they work well enough. Words can be difficult.

John Platko wrote:And if modern science can't shed any definitive light on whether the ball falls continuously or discreetly, can ancient science?

You're suggesting that modern science has somehow ignored earlier theories in the development of current theories?

It's possible. It would be understandable if Newton just used Aristotle's work on physics to prop open a door or something. After reading how heavier objects fall faster than lighter ones and that an object only continues to move if a force is being applied to it Newton might have decided to make the book useful by chucking in the fireplace for all I know. But perhaps not.

Aristotle did have some great ideas along with his clunkers. His thought about the impossibility of traversing an infinite discreet series may be one of them.

Sendraks wrote:Well quite.

Where "learning" actually means "ignoring what could be learn for the sake of not understanding and attempting to drive woo shaped wedges into science."

woo?

what woo?

John Platko wrote:
By true I mean: having all the expected or necessary qualities.

You mean by matching every possible description under every possible circumstance?
And that should there be an instance of a theory not matching up, it is immediately considered false?

What a curiously digital and unscientific mind you have.

John Platko wrote:It's possible.

Many things are possible. We like to cite evidence however, rather than speculate or make unevidenced assertions.

John Platko wrote:woo?

what woo?

That which lurketh underneath your obviously transparent lines of inquiry.

having fun with zeno's paradox #935:

How true? About as true as tape measure. Now go back to school for Christ sake. I do not want to live with people like you around.

Sendraks wrote:

John Platko wrote:
By true I mean: having all the expected or necessary qualities.

You mean by matching every possible description under every possible circumstance?
And that should there be an instance of a theory not matching up, it is immediately considered false?

What a curiously digital and unscientific mind you have.

I mean having a theory match reality. But I'm not one to be petty, I find it easy to get passed the wobbly way Newton's law works for planets but downright unsettling that it predicts an infinite number of changes in energy as a golf ball falls from my hand, especially since that is unsubstantiated and rather difficult to understand. And I'll point out, not just for me, because no one here has been able to shed any light on just how many changes in energy a golf ball experiences when if falls a fixed distance.

John Platko wrote:It's possible.

Many things are possible. We like to cite evidence however, rather than speculate or make unevidenced assertions.

Actually, I did cite my evidence for how ancient science might have something useful to add to this question, Aristotle's theory:

His thought about the impossibility of traversing an infinite discreet series may be one of them.

John Platko wrote:woo?

what woo?

That which lurketh underneath your obviously transparent lines of inquiry.

Your statements lead me to conclude that which lurketh underneath my lines of inquiry to be opaque to you. But by all means, have a go at revealing what is at the depths of my inquiry, I think I would find it amusing.

I'm mostly interested in understanding if it's reasonable to infer an infinite number of changes in velocity and momentum as a golf ball falls. And if not, why not.

The obvious fact uncovered by this thread that Newton's laws are really only metaphors, mathematical metaphors which in some ways are similar to the actual physical phenomena and in other ways are not, and yet in other ways are undetermined is interesting. But I'd rather know how the ball really falls.

newolder wrote:having fun with zeno's paradox #935:

My question has nothing to do with Zeno's paradox. Me thinks you don't understand my question.

And you don't have to take my word for it.

Cito di Pense wrote:

John Platko wrote:my issue is not the same as his.

The golf ball's momentum is quantized, but the quanta are small relative to the total momentum. Indeed, if you can take that into account, your issue is much different to Zeno's.

tuco wrote:How true? About as true as tape measure. Now go back to school for Christ sake. I do not want to live with people like you around.

Does that mean you can't answer my question either?

I answer: They're really Newton's Metaphors of Motion.

I answered your question. Have you ever used tape measure? Did you ask how "true" the measurement you made was? About as true as, the results you get from, Newton laws. Go back to school, seriously.

John Platko wrote:
My question has nothing to do with Zeno's paradox. Me thinks you don't understand my question.

Ok.

His thought about the impossibility of traversing an infinite discreet series may be one of them.

Infinite series are cute.

And you don't have to take my word for it.

Cito di Pense wrote:

John Platko wrote:my issue is not the same as his.

The golf ball's momentum is quantized, but the quanta are small relative to the total momentum. Indeed, if you can take that into account, your issue is much different to Zeno's.