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PostPosted: Tue Feb 14, 2012 22:00 
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I usually think of closed systems as being a tool where you use an idealised framework to test your working of the problem. They are all a part with frictionless surfaces and lossless transfers.

Real world examples are often far too complex to close. You take the equations developed in the model and apply them knowing that they will reveal the transfers of energy and momentum between the parts of the system you are interested in.

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PostPosted: Tue Feb 14, 2012 22:52 
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dcbwhaley wrote:
I have asked Steve to PM me the questions he wishes me to answer so that I consider them at leisure, without the smokescreen he keeps raising. Then I will present my answers openly in the forum.

That's too funny! :lol:
Hey everyone, let's instead PM each other our responses on the "open" debates that we have, that really will encourage "honest" debate.

Speaking of "smokescreens" ...
dcbwhaley wrote:
Quote:
Certain allowance for off-topic-ness is tolerated but not if it is to divert the discussion.

Really? If a brief aside about field momentum in a discussion about conservation laws is not to be tolerated then you have a very sterile idea of what constitutes a good debate.

Misrepresentation!
Your brief aside was "without suggesting that they were relevant to the substantive argument"; you have a very 'sterile' idea of what constitutes a diversion, and smokescreens.

dcbwhaley wrote:
Precision is excellent but before you start doing precise calculations you have to make sure that you are solving the correct question

"it is sometimes more convenient not to"

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PostPosted: Tue Feb 14, 2012 23:09 
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In that case please list your questions in a post in open forum. Just the questions, no snide remarks, no aspersions on my intelligence. Just the questions. Then I will answer them if I can.

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PostPosted: Tue Feb 14, 2012 23:15 
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Toltec wrote:
Real world examples are often far too complex to close.


Not for Steve they aren't :-) He always closes his system. In fact he doesn't seem to understand what an unclosed system is or why you would choose to use one.

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PostPosted: Tue Feb 14, 2012 23:39 
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dcbwhaley wrote:
I have asked Steve to PM me the questions he wishes me to answer so that I consider them at leisure, without the smokescreen he keeps raising. Then I will present my answers openly in the forum.
You like everyone has the option to take all the time they wish and then post when they choose. It is completely pointless to ask questions privately so that you can consider and post later.
dcbwhaley wrote:
SafeSpeedv2 wrote:
I see it that Steve has shown extreme patience in carefully explaining his points clearly
Posts like the last one with massive multilevel quoting, awkward cross references and a continued insistence that he has answered my questions when he hasn't are the antithesis of clarity.
That may well be subjective. I find the quoting posts (especially when the person being quoting is clearly repeated properly), really very easy to 'read' indeed. It is there 24/7, and if you have a problem with comprehension of the posts then all the admin will help all they can.
Perhaps each point to be discussed needs it's own thread, although that does seem a bit un-necessary.
dcbwhaley wrote:
SafeSpeedv2 wrote:
it seems normal to me that precision is the name of the game.
Precision is excellent but before you start doing precise calculations you have to make sure that you are solving the correct question
But the original posters question is about how to ensure 'M exists' is a suitable talking point that will lead to greater road safety. I appreciate that discussing momentum is an interesting subsection within this (that) thread.

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PostPosted: Wed Feb 15, 2012 00:47 
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dcbwhaley wrote:
Just the questions, no snide remarks, no aspersions on my intelligence. Just the questions. Then I will answer them if I can.

You initiated the snide remarks and aspersions (proven without rebuttal); you can't claim the moral high ground here.

Yet just one post prior:
dcbwhaley wrote:
Toltec wrote:
Real world examples are often far too complex to close.


Not for Steve they aren't :-) He always closes his system. In fact he doesn't seem to understand what an unclosed system is or why you would choose to use one.
[my bold]

That more misrepresentation, as well as a smokescreen, as well as casting aspersions on my intelligence.
You ask me to refrain from behaviour (that you initiated), that you then commit yourself in your very next post 6.5 minutes later - astounding!

The understanding has been repeated (without rebuttal) here. I need not choose to use one in the idealised systems we have been discussing; all were properly closed and explained.
Moreover, why should I have left them unclosed when you asked me how I would close them? "where has its momentum been conserved?"; "how does conservation of momentum apply to..."

Will this behaviour ever stop?

dcbwhaley wrote:
In that case please list your questions in a post in open forum.

Are you not able to simply read the questions in the given post? Need I really repeat them for a seventh time?


However, I will call your bluff. See my next post, which will merely be a copy n paste from previous ones !!

Just the answers, no snide remarks, no aspersions on my intelligence. Just the answers:
(and don't PM them to me)

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PostPosted: Wed Feb 15, 2012 01:03 
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dcbwhaley wrote:
Energy is always conserved. Momentum isn't.


Steve wrote:
...By the same token, I can show that in a non-closed system, energy also isn't conserved (external power source, or even the same external force you keep referring to (power=force*velocity)).
I can apply an external power into a system (analogous to an external force for momentum), so can I now make the umbrella claim, in just the same way that you had, that "energy isn't conserved"?


Steve wrote:
Of course you could have rightly said that energy is conserved in a closed system and momentum not conserved in a non-closed system. However, it would have been just as correct to say that energy is not conserved in a non-closed system and momentum conserved in a closed system ... but it is somewhat pointless, unless with that I can equally say, without any further qualification: "Momentum is always conserved. Energy isn't."? Yes or no?


Steve wrote:
Remember Dave, it was you, and only you, who made the distinction between 'conservation of energy' and 'conservation of momentum' by quite equivocally saying "is" and "isn't" respectively. Critically, there was no open or closed clause in your original claim: "Energy is always conserved. Momentum isn't." and that's it, with no reference to how or what either system is.

[...]

Can you provide your description of the circumstances where momentum isn't conserved whilst energy is?


Steve wrote:
Thus it doesn't matter how you defined 'closed', so long as it is applied equally to both concepts. Then your argument ... inherently fails. True or false?

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PostPosted: Wed Feb 15, 2012 09:45 
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dcbwhaley wrote:
Toltec wrote:
Real world examples are often far too complex to close.


Not for Steve they aren't :-) He always closes his system. In fact he doesn't seem to understand what an unclosed system is or why you would choose to use one.


I am quite certain he does understand, from his last post it is clear he is objecting to the way you appear to have mixed closed and unclosed in a single statement. What is also clear from past discussions is that Steve requires precision and you will never admit that you phrased something loosely and the argument descends to semantic quibbling.
It may be possible to come up with a classical system where energy is conserved and momentum is not, why you would want to I am not sure.

ETA - yes I know, glass houses and all that. A particular discussion about evolution, mutation and natural selection springs to mind. :)

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PostPosted: Wed Feb 15, 2012 10:16 
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Toltec wrote:
clear from past discussions is that Steve requires precision and you will never admit that you phrased something loosely and the argument descends to semantic quibbling.

I freely admit that, in non-technical discussions such as the one that started this exchange, I often phrase things loosely.

Quote:
It may be possible to come up with a classical system where energy is conserved and momentum is not,

As I did in my last thought experiment - the vehicle charging the battery to slow down.

Quote:
why you would want to I am not sure.

For easier calculation usually. For example when calculating the trajectory of a satellite passing a planet you could calculate the new path either by treating the planet/satellite as a closed system and using CofLM or by considering the satellite alone as an open system with a external applied force - gravity - Lm not being conserved. Both approaches are good Physics but the second can be easier.

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PostPosted: Wed Feb 15, 2012 10:37 
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Steve wrote:
I can equally say, without any further qualification: "Momentum is always conserved. Energy isn't."?

You are quite free to say that. Your listener will either understand the circumstances in which that is true or will ask you - politely I hopev- to explain those circumstances to him.

Quote:
Critically, there was no open or closed clause in your original claim: "Energy is always conserved. Momentum isn't." and that's it, with no reference to how or what either system is.

I though that in the context - consequence of RTA -it was clear that i was referring to a non closed system, a single motor vehicle. I apologize idf that was not sufficently obvious.

Quote:
Can you provide your description of the circumstances where momentum isn't conserved whilst energy is?


Consider this thought experiment. My system consist of a wooden cab mounted on four wheels on two solid axles. One of ther axles is connected via a gearbox to an electrical motor/generator machine whose terminals can be, through suitable control gear, connected to or disconnected from a secondary battery. As this is thoughtthere is no friction in the mechanisms , no air resistance and no losses in the gearbox. The battery and charging gear are also perfectly lossless. The whole contraption weigh 1000kgm.

At the start of the experiment it is travelling at 10mS along a perfectly smooth surface and the electrical machine is not connected to the battery. Thus the momentum is, with suitable choice of axis, equal to 10^4 kg m/s and the kinetic energy is 5*10^4 joules.

At some point, by unspecified means, the electrical machine is connected, as a generator, to the battery. The kinetic energy of the contraption is converted to electrical energy which is stored in the battery and it comes to rest. As the conversion process is perfect, energy has been conserved but Its momentum is now zero so momentum has not been conserved. The reason that momentum is not conserved is because the system is not closed and an external force has been applied through the wheels. It is easily shown that the value of that force is equal to the rate of change of momentum.

By further unspecified means the battery is reconnected to the electrical machine, now acting as a motor, and the contraption accelerates until all the electrical energy is reconverted to kinetic energy when its speed will again be 10m/s.

Quote:
Thus it doesn't matter how you defined 'closed', so long as it is applied equally to both concepts. Then your argument ... inherently fails. True or false?

False as demonstrated above.

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PostPosted: Wed Feb 15, 2012 11:48 
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dcbwhaley wrote:
For easier calculation usually. For example when calculating the trajectory of a satellite passing a planet you could calculate the new path either by treating the planet/satellite as a closed system and using CofLM or by considering the satellite alone as an open system with a external applied force - gravity - Lm not being conserved. Both approaches are good Physics but the second can be easier.


Aha, so what we could really say is that within the model you are using momentum is not conserved. You can do this because transfer of momentum out of the system has no significant effect on the result.

So what you are choosing to do is conserve energy but not momentum within your system of calculations.

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PostPosted: Wed Feb 15, 2012 12:08 
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dcbwhaley wrote:
Toltec wrote:
clear from past discussions is that Steve requires precision and you will never admit that you phrased something loosely and the argument descends to semantic quibbling.

I freely admit that, in non-technical discussions such as the one that started this exchange, I often phrase things loosely.

The phrase in question was quite unequivocal; one always is, another isn’t. For once, detail wasn’t a problem.

dcbwhaley wrote:
Quote:
It may be possible to come up with a classical system where energy is conserved and momentum is not,

As I did in my last thought experiment - the vehicle charging the battery to slow down.

Your system was incomplete. There is no correct and complete example where the ground doesn’t accelerate (there is a force acting on it), thus you cannot claim conservation is not conserved. Using your same logic, I can also show how energy isn’t conserved: the vehicle is solar powered and uses a light bulb to dissipate the transferred kinetic energy.
You can close the variables of any system, idealised or real-world, if the variables are known. It is easy to do once you know how to resolve them.

dcbwhaley wrote:
Quote:
why you would want to I am not sure.

For easier calculation usually. For example when calculating the trajectory of a satellite passing a planet you could calculate the new path either by treating the planet/satellite as a closed system and using CofLM or by considering the satellite alone as an open system with a external applied force - gravity - Lm not being conserved. Both approaches are good Physics but the second can be easier.

It might well be more convenient to use a wrong or incomplete model, but doing so doesn’t invalidate the fundamental law of conservation.
Your ignoring the law for convenience doesn’t mean the law isn’t; it merely means you ignored it.

Energy is always conserved. Momentum is always conserved.

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PostPosted: Wed Feb 15, 2012 12:55 
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Toltec wrote:
Aha, so what we could really say is that within the model you are using momentum is not conserved. You can do this because transfer of momentum out of the system has no significant effect on the result.
So what you are choosing to do is conserve energy but not momentum within your system of calculations.


More correctly: I am choosing a system in which energy is conserved but momentum is not. Steve denies that such a system exists and he may be right but he hasn't yet refuted my example

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PostPosted: Wed Feb 15, 2012 13:03 
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dcbwhaley wrote:
Steve wrote:
I can equally say, without any further qualification: "Momentum is always conserved. Energy isn't."?

You are quite free to say that.

The question is: can it be justified using your same logic of “conveniently” defining the system?

dcbwhaley wrote:
Quote:
Critically, there was no open or closed clause in your original claim: "Energy is always conserved. Momentum isn't." and that's it, with no reference to how or what either system is.

I though that in the context - consequence of RTA -it was clear that i was referring to a non closed system, a single motor vehicle. I apologize idf that was not sufficently obvious.

It was clear, one is always conserved, the other isn’t. There was no other clause.
I have shown how energy also wouldn’t be conserved in that same defined system by applying your same logic (external energy), thus again proving your original statement to be untrue. Yes or no?

dcbwhaley wrote:
Quote:
Can you provide your description of the circumstances where momentum isn't conserved whilst energy is?

Consider this thought experiment...

Again, you dismiss the fact that the surface is moving because of the force acting upon it. Thus your model is wrong or incomplete. True or false?

If your model was complete, momentum and energy are conserved. True or false?
If your model was incomplete, momentum and/or energy may not be conserved; true or false?

Is it right to say that a law isn’t so on the basis of a knowingly incomplete model?

dcbwhaley wrote:
Quote:
Thus it doesn't matter how you defined 'closed', so long as it is applied equally to both concepts. Then your argument ... inherently fails. True or false?

False as demonstrated above.

You didn’t apply the definition “equally to both concepts”. My corollary to your example (transfer of light energy) is an example where the definition of ‘closed’ (vehicle, planet surface) can be equally applied, thus ignoring all else outside of this boundary “for convenience”.

Hence you didn’t answer my question.

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PostPosted: Wed Feb 15, 2012 13:16 
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Steve wrote:
Your system was incomplete.

The system is what I choose it to be, so is is complete. If I find it more convenient to use a system which transfers momentum outwards because an external force is acting, rather than a larger system which conserves momentum in the absence of an external force, that I can do so.

Quote:
There is no correct and complete example where the ground doesn’t accelerate (there is a force acting on it), thus you cannot claim conservation is not conserved.

I can claim that Moon is made of Cheddar cheese if I want to. But by not including the ground in my system then it is a non conserving system.

Quote:
Using your same logic, I can also show how energy isn’t conserved: the vehicle is solar powered and uses a light bulb to dissipate the transferred kinetic energy.

If that is the most convenient model for you them feel free to do so. I won't argue with you.

Quote:
You can close the variables of any system, idealised or real-world, if the variables are known.

Indeed you can. But you need not.

Quote:
It might well be more convenient to use a wrong or incomplete model but doing so doesn’t invalidate the fundamental law of conservation.
Your ignoring the law for convenience doesn’t mean the law isn’t; it merely means you ignored it.
Energy is always conserved. Momentum is always conserved.


The law of conservation of linear momentum does not state "Momentum is always conserved" it says, and you drive me to shouting in colour :) " if no external force acts on a closed system of objects, the momentum of the closed system remains constant."
That does not insist the momentum of a closed system remains constant. It allows an external force to change. We use that fact continually in engineering and everyday life

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PostPosted: Wed Feb 15, 2012 13:22 
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dcbwhaley wrote:
More correctly: I am choosing a system in which energy is conserved but momentum is not. Steve denies that such a system exists

It cannot exist as you have chosen to say, purely for “convenience”, that other existing variables do not exist. In reality, they always exist and you can’t get around them. All you can do is shoose and hope they are not significant. Not being significant doesn't mean there isn't conservation.

dcbwhaley wrote:
and he may be right but he hasn't yet refuted my example

You are again misrepresenting. One of the evaded questions regarding the equally applied “system” still needs to be addressed.

With a very small tweak of the boundaries, I can easily show a non-closed system where momentum is conserved but energy isn’t.

Consider the following thought experiment. A small self-powered (battery, ICE) moving vehicle uses a normal friction brake to dissipate the kinetic energy via (at least partly) thermal radiation (just like our original example). If “conveniently” choosing the system to be the planet and the vehicle, momentum is conserved but energy is varied.
Thus I can now make the bold statement, relating to your original example, that “momentum is always conserved; energy isn’t.”, yes or no?

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PostPosted: Wed Feb 15, 2012 13:25 
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Steve wrote:
Your system was incomplete.

The system is what I choose it to be, so is is complete. If I find it more convenient to use a system which transfers momentum outwards because an external force is acting, rather than a larger system which conserves momentum in the absence of an external force, that I can do so.

Quote:
There is no correct and complete example where the ground doesn’t accelerate (there is a force acting on it), thus you cannot claim conservation is not conserved.

I can claim that Moon is made of Cheddar cheese if I want to. But by not including the ground in my system then it is a non conserving system.

Quote:
Using your same logic, I can also show how energy isn’t conserved: the vehicle is solar powered and uses a light bulb to dissipate the transferred kinetic energy.

If that is the most convenient model for you them feel free to do so. I won't argue with you.

Quote:
You can close the variables of any system, idealised or real-world, if the variables are known.

Indeed you can. But you need not.

Quote:
It might well be more convenient to use a wrong or incomplete model but doing so doesn’t invalidate the fundamental law of conservation.
Your ignoring the law for convenience doesn’t mean the law isn’t; it merely means you ignored it.
Energy is always conserved. Momentum is always conserved.


The law of conservation of linear momentum does not state "Momentum is always conserved" it says, and you drive me to shouting in colour :) " if no external force acts on a closed system of objects, the momentum of the closed system remains constant."
That does not insist the momentum of a closed system remains constant. It allows an external force to change.
Similarly the law of conservation of energy states "the total amount of energy in an isolated system remains constant over time" That does not preclude energy being transferred between systems.

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PostPosted: Wed Feb 15, 2012 13:58 
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dcbwhaley wrote:
Steve wrote:
Your system was incomplete.

The system is what I choose it to be, so is is complete.

How can you choose to have a “complete” system which is affected by an external influence?

dcbwhaley wrote:
If I find it more convenient to use a system which transfers momentum outwards because an external force is acting, rather than a larger system which conserves momentum in the absence of an external force, that I can do so.

Yes you can, but momentum is still conserved, just as much as energy is. This is where your argument outright fails.

dcbwhaley wrote:
I can claim that Moon is made of Cheddar cheese if I want to. But by not including the ground in my system then it is a non conserving system.

But by not including where the thermal energy of the brake is going, then it is a doubly non-conserving system (energy and momentum). True or false?

How in your system do you keep the ground from accelerating if there is a force acting upon it? If you choose to ignore additional momentum of the ground, haven't you also ignored F=ma? Now I know I don't need to strawman by asking you if f=ma is always true - do I?

dcbwhaley wrote:
Quote:
Using your same logic, I can also show how energy isn’t conserved: the vehicle is solar powered and uses a light bulb to dissipate the transferred kinetic energy.

If that is the most convenient model for you them feel free to do so. I won't argue with you.

So it is true, that for your example where you said energy is always conserved, I have shown that energy isn’t conserved?

Remember that you didn’t define your system within your original claim, hence making any such claim, let alone claiming one is true and the other false, is not reasonable.

dcbwhaley wrote:
That does not insist the momentum of a closed system remains constant.

dictionary.com wrote:
A principle stating that the total linear momentum of an isolated system remains constant regardless of changes within the system.

wiki wrote:
In an isolated system (one where external forces are absent) the total momentum will be constant


You presed the quote button instead of edit

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PostPosted: Wed Feb 15, 2012 16:00 
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This argument is now about semantics rather than Physics so I shall...
___________________________________________________________________

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PostPosted: Wed Feb 15, 2012 16:46 
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dcbwhaley wrote:
This argument is now about semantics rather than Physics so I shall...
___________________________________________________________________


Thanks ,gents , before the GOOD name of SS gets besmirched .Thats what I suggrested several posts previous .
So SHAKE HANDS & call it a draw .

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