Newton's first law

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  • #51
WannabeNewton
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I'm not saying work must be performed just that there must be potential or kinetic energy for a force to be present.
A book at rest on a table has no potential nor kinetic energy if I pick my potential energy reference as the table the book rests on (assuming the book has negligible thickness).
 
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A book at rest on a table has no potential nor kinetic energy if I pick my potential energy reference as the table the book rests on (assuming the book has negligible thickness).
Reference frame shouldn't matter because you still have potential energy from gravity between the book and presumably earth or at least the table. Am I really missing something here? Seems to me like the only time this would ever be the case is in a closed system where we are ignoring potential energies and stating there is some outside force. But that isn't really practical.

I could argue oscillating particles for the book on a table but since we're already looking at an infinitely thin book it seems fair to concede this isn't a realistic scenario.

I suppose my problem lies in the idea that forces can come from nothing instantaneously but that doesn't sit we'll with me intuitively (which isn't to say i'm right).
 
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Newtons first and second laws relate to resultant (or Net) forces

This is posted to clear up any misunderstanding that my original post also included the third law. I made no comment about the third law!
 
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Dale
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This is posted to clear up any misunderstanding that my original post also included the third law. I made no comment about the third law!
You said "Newton's laws" in post 31 and 32. Newton's 3rd law is one of Newton's laws, so your statement could easily be understood incorrectly by a novice reader to have been intended to apply to Newton's 3rd law. This is why in post 33 I said merely that your comments were "overstated", not that it was "wrong". The statement went too far, as I explained in considerable detail.

It is easy to accidentally overstate something, I have done it many times myself. The proper response when someone points out a miscommunication is "oops, I didn't mean to say that". I have had to do that many times, it doesn't hurt very much.
 
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D H
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Thanks to D H and DaleSpam as I am one of those that missed the point of the first law and always saw it as a wordier way of expressing Newton's second law.
They say different things to me. The first says what happens when no forces are present. It's a null statement. The second says what happens when forces are present. It is consistent with the first in the special case of a null force. It has to be consistent with the first; any viable theory must necessarily be internally consistent or it is not viable.

That of course is viewing Newton's first two laws as laws of nature. Here's another rather different way of looking at them: They are not laws of nature. They are instead definitions. The first law defines the concept of "inertial frames". The second law defined the concept of "force".

From this more modern perspective, it's only the third law and Newton's first two corollaries that are laws of nature. Those first two corollaries, in modern parlance, state that forces are vectors and hence add as vectors. Another way to put it: Forces are subject to the superposition principle. Some instructors even teach the superposition principle as "Newton's fourth law".

That's a bit of a mis-statement of course; Newton's Principia only identifies three laws (plus a bunch of corollaries). Nobody in their right mind wants to teach those two corollaries as-is. They're worded archaically and geometrically. It's much easier to teach the algebraic superposition principle.



Reference frame shouldn't matter because you still have potential energy from gravity between the book and presumably earth or at least the table. Am I really missing something here?
Yes. You are missing that potential energy contains an arbitrary constant. Alternatively, one can arbitrarily select a point at which potential energy is zero.

I suppose my problem lies in the idea that forces can come from nothing instantaneously but that doesn't sit we'll with me intuitively (which isn't to say i'm right).
One way to look at forces is not to look at forces at all. Look at energy instead. That's what you're going to see if you stay in physics beyond the freshman/sophomore intro to physics series of classes. Another way to look at forces (conservative forces) is that they are the gradient of some potential energy function. Forces don't arise from energy; they arise from how energy changes with respect to position. Notice that that arbitrary constant becomes irrelevant here as two smooth functions that differ only by an arbitrary constant have the same gradient everywhere.
 

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