Re: defining laws in isolation or 'no other force acting'
<hibbsa-/[email protected]> 18 Jan 2014 23:26:32 -0800
| Newsgroups | gmane.science.physics.fabric-of-reality |
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| Message-ID | <[email protected]> |
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<p> <br>
<br>
---In [email protected], <david.deutsch@...> wrote:<br>
<br>
> On 6 Jan 2014, at 12:23, <hibbsa@... mailto:hibbsa@...> <hibbsa@... mailto:hibbsa@...> wrote:<br>
><br>
>> ---In [email protected] mailto:[email protected], <david.deutsch@...> wrote:<br>
>> <br>
>>> In that sense, the laws of constructor theory *are* about isolated systems: the substrate plus constructor.<br>
>> <br>
>> The isolated system doubles as a better way to make clear there aren't other influences acting. Certainly there are applications where the boundary has independent value....say in how the major problems of mechanical engineering were solved (which were fundamental in nature)<br>
>> <br>
>> But, take Newton's laws of motion. Say, an object will remain at rest or continue the way its going at constant velocity, WHEN no other forces are acting. <br>
>> <br>
>> Does that say that when forces are acting, inertia and the other laws aren't also acting? No one would say that. Newton uses forces acting and that law in combination to get orbital behaviour, show conservation of momentum hence explain elliptical orbit and so on. <br>
><br>
> Newton's first law specifies the behaviour of isolated systems.<br>
><br>
> Newton's second and third laws specify the behaviour of all systems, isolated or not. Indeed, the first law follows from the second, if one adopts Newton's conception of what it means for a force to 'act'.<br>
<br>
Hi DD, Please excuse the delay responding to the point you made here. I had never thought of it before, so have had to spend some time thinking about this matter of the first law following from the second. <br>
<br>
I might interpret this wrong, but the easiest way I can see you are right, starts with setting F=m.a = 0 (m !=0) then looking for conditions consistent with this state. From this, there is a chain of inferences - as rigorous as you like - that will, as you say, lead to the first law. <br>
<br>
Whether this is trivial or non-trivial, is much more problematic, because the reasoning seems to reverse quite easily, such that the second law derives from the first. In both cases, it is necessary to have fore knowledge of both 'velocity' and 'acceleration' and the relationship between them. <br>
<br>
One direction may be more or less intuitive than the other, but other than that it's hard not to get a draw between them. Reversibility doesn't make something trivial, but it does make the question of triviality or not, dependent on what significance you seek to attach in the first place. It's hard to attach significance of the form 'the first law follows the second' non-trivially, when there is reversibility. <br>
<br>
Or maybe I've completely misconceived the derivation you use. I wait in hope of your response as ever. <br>
<br>
<br>
<br>
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