Momentum - self-dissipative vs resistance

[Kiril]

Hello everyone,

Recently I realized what momentum refers to in reality. Briefly:
On the perceptual level it refers to the phenomenon which enables an entity to continue motion even after the cause of the initial motion, either by direct contact or via field, no longer affects it. It is distinct from the concepts of motion and velocity(a rate) in that it describes that cause/force 'within-it-self'(a transient attribute)allowing it to continue motion(which is why it is proportional to mass).

This is such a common aspect of daily experience that I've never thought of it(even with the definition of inertia in-mind) as a separate phenomenon requiring explanation - that is, that this motion is not a necessary consequence; one could just as well expect the motion not to continue.

And with this realization came the following questions, I'd appreciate help with:
1 - Is it experimentally demonstrated that this attribute is not self-dissipative(perhaps in the form of some radiation...) as apposed to being purely resisted by opposing forces(air resistance, surface friction) - that is, if it is not a mixed case?
And if so, doesn't this imply the possibility of perpetual motion?

2 - What makes it possible for an object to continue this motion?

Thanks.

 

[Drakkith]

I think your description more accurately fits Inertia. The reason being that any object resists any change in its motion, which is pretty much what inertia is. Without inertia, an object could still continue to move as long as there was no force applied to it. As to WHY should it work this way, the only possible answer I can come up with is that it simply DOES work this way.

 

[russ_watters]

1 - Is it experimentally demonstrated that this attribute is not self-dissipative(perhaps in the form of some radiation...) as apposed to being purely resisted by opposing forces(air resistance, surface friction) - that is, if it is not a mixed case?
And if so, doesn't this imply the possibility of perpetual motion?

Yes...just be careful to be precise about what you mean by "perpetual motion". There's a lot of misconceptions out there about what the term means. I suggest you look it up to make sure it means what you think it means.

2 - What makes it possible for an object to continue this motion?

The lack of anything to change it.

Newton's first law: "The velocity of a body remains constant unless the body is acted upon by an external force."
http://en.wikipedia.org/wiki/Newton's_laws_of_motion

 

[Kiril]

I think your description more accurately fits Inertia. The reason being that any object resists any change in its motion, which is pretty much what inertia is.

I think inertia describes the general category of such phenomena, in which momentum is subsumed and also provides a quantitative measure of.

Without inertia, an object could still continue to move as long as there was no force applied to it.

How do you know?
It seems to me that without that mechanism which makes inertia possible, motion would cease as soon as the influence of the impetus ceases.
I just want to make clear that by "why" I mean, what physical mechanism.


Thanks

 

[Kiril]

Yes...just be careful to be precise about what you mean by "perpetual motion". There's a lot of misconceptions out there about what the term means. I suggest you look it up to make sure it means what you think it means. The lack of anything to change it.

Newton's first law: "The velocity of a body remains constant unless the body is acted upon by an external force."
http://en.wikipedia.org/wiki/Newton's_laws_of_motion

If I understand correctly, this is describing the effects of inertia, not the causes. This is precisely what dawned on me: inertia/momentum must be a product of some underlying mechanism(not directly perceivable, like the cause of gravity) which has an interface to the macro/perceivable mechanics of objects - the level at which Newton was working to describe.
Its undeniably true that the vector sum of the forces, of inertial motion, is equal to zero - but this does not mean the absence of the action of forces(balanced); just as the absence of motion of a suspended weight does not negate the existence of Gravity. Nor that every force requires a constant energy source.

 

[Drakkith]

I think inertia describes the general category of such phenomena, in which momentum is subsumed and also provides a quantitative measure of.


How do you know?
It seems to me that without that mechanism which makes inertia possible, motion would cease as soon as the influence of the impetus ceases.
I just want to make clear that by "why" I mean, what physical mechanism.


Thanks

The definition of inertia is resistance in change to motion. If inertia dissappeared then any force would accelerate any object to infinity. So in this hypothetical situation everything would act in a similar way to light. But if you disagree then that's perfectly ok, as inertia isn't going away so there isn't any real point to debating what would happen IF it did. Also, it is my understanding that Mass is the mechanism that causes inertia. (Or rather that inertia is proportional to mass) So if you remove the mass, you have light! (Or maybe absolutely nothing)

If I understand correctly, this is describing the effects of inertia, not the causes. This is precisely what dawned on me: inertia/momentum must be a product of some underlying mechanism(not directly perceivable, like the cause of gravity) which has an interface to the macro/perceivable mechanics of objects - the level at which Newton was working to describe.
Its undeniably true that the vector sum of the forces, of inertial motion, is equal to zero - but this does not mean the absence of the action of forces(balanced); just as the absence of motion of a suspended weight does not negate the existence of Gravity. Nor that every force requires a constant energy source.

Nothing here describes the effect we see. In fact, it is directly opposite of what we have observed; In that you MUST apply a force to cause a change in motion. Nothing anywhere in Science says that something must cause an object NOT to accelerate.

 

[Kiril]

The definition of inertia is resistance in change to motion. If inertia dissappeared then any force would accelerate any object to infinity. So in this hypothetical situation everything would act in a similar way to light. But if you disagree then that's perfectly ok, as inertia isn't going away so there isn't any real point to debating what would happen IF it did. Also, it is my understanding that Mass is the mechanism that causes inertia. (Or rather that inertia is proportional to mass) So if you remove the mass, you have light! (Or maybe absolutely nothing)

I can understand your example, however, should it not also follow, from said definition, that given an initial impetus, and after loosing contact with the point of application, it is the property of inertia which insists that the motion continue; which resists a change in motion(to decelerate)?
And this tendency or power to continue is what momentum measures?
And in the absence of inertia, would not occur?

BTW, when I refer to cases without inertia, I mean only to isolate specific consequences - it should not be seen as attempt to form conclusion from fictitious, a priori conditions.

Nothing here describes the effect we see. In fact, it is directly opposite of what we have observed; In that you MUST apply a force to cause a change in motion. Nothing anywhere in Science says that something must cause an object NOT to accelerate.

I don't understand how this relates to the paragraph, to which it is a response?
What I was trying to say, thinking for myself and not relying only on 'science', is that inertial motion must involve a force(s), which enables the motion to continue past the point of application.

 

[Drakkith]

I can understand your example, however, should it not also follow, from said definition, that given an initial impetus, and after loosing contact with the point of application, it is the property of inertia which insists that the motion continue; which resists a change in motion(to decelerate)?
And this tendency or power to continue is what momentum measures?
And in the absence of inertia, would not occur?

I'm not an expert, so all I can say is that my understanding of how it works is that an object requires no force to continue in uniform motion.

I don't understand how this relates to the paragraph, to which it is a response?
What I was trying to say, thinking for myself and not relying only on 'science', is that inertial motion must involve a force(s), which enables the motion to continue past the point of application.

And I'm saying that to my knowledge science says that an object requires no force to keep moving. Looking at the law of inertia it specifically states that. I can easily see how it would seem that a force would be required to continue to move, however this view has been obsolete for centuries I believe. We have yet to discover any force that keeps an object in motion through space. If you look around the everyday world, this easily seems to not be the case. It is only once we look closer that we realize the only thing that stops an object from moving is a force such as friction, resistance, or similar. In fact we need MORE force than required by inertia to accelerate an object here on Earth because of those things that oppose motion.

 

[Kiril]

We have yet to discover any force that keeps an object in motion through space. If you look around the everyday world, this easily seems to not be the case. It is only once we look closer that we realize the only thing that stops an object from moving is a force such as friction, resistance, or similar. In fact we need MORE force than required by inertia to accelerate an object here on Earth because of those things that oppose motion.

Thank you for clarifying.
That force is inertia. What else do you think your measuring with momentum?
I looked and it doesn't seem obvious to me how objects continue moving past the point of application(only that they do).
Its why I asked,initially, for experimental evidence that an object would continue indefinitely(vs a very long distance), since it seems like Newton made an a priori assumption(or approximation) in forming the first law.

In any case, I appreciate our dialogue since I'm now much more clear on my own position.

Thanks,

Kiril

 

[walkermn]

nice piece if advice.good information

 

[russ_watters]

Thank you for clarifying.
That force is inertia. What else do you think your measuring with momentum?

Force. Inertia. Momentum.

No two of these three are the same thing. Force is not inertia. Momentum is not inertia.

 

[sophiecentaur]

It could be worthwhile pointing out that Inertia, as has been defined and used in this thread, seems to relate more to the quantity Mass than anything else. (Possibly Inertial Mass) Inertia is certainly not a Force and, Momentum, not being a Force either, doesn't actually resist a force (only 'like' quantities could add, surely). Force will change Momentum, of course. If we're talking Newtonian matters then the Second Law of Motion says it all.
The issue of 'dissipation' is another matter which is a lot harder to sort out.

 

[Drakkith]

Thank you for clarifying.
That force is inertia. What else do you think your measuring with momentum?
I looked and it doesn't seem obvious to me how objects continue moving past the point of application(only that they do).
Its why I asked,initially, for experimental evidence that an object would continue indefinitely(vs a very long distance), since it seems like Newton made an a priori assumption(or approximation) in forming the first law.

In any case, I appreciate our dialogue since I'm now much more clear on my own position.

Thanks,

Kiril

Experimental evidence? The Earth has been going around the Sun for 4 1/2 billion years! There's your evidence right there. If 4+ billion years isn't a long enough time for you, then you will never find an acceptable answer.

As Russ and Sophie pointed out, neither force, inertia, nor momentum are the same thing. It is extremely important that you understand the difference in basic concepts such as these or things will not make sense. And perhaps just as importantly, WHY the differences exist. I am not fluent enough nor knowledgeable enough to help you any further, so I will leave that to someone else.

 

[sophiecentaur]

There IS this Dark Matter thing which could, I feel, be put down to non-conservation of Momentum under some circumstances rather than the existence of other 'stuff' we haven't seen yet.

 

[Kiril]

Force. Inertia. Momentum.

No two of these three are the same thing. Force is not inertia. Momentum is not inertia.

Could you reduce the difference between inertia and momentum to the perceptual level of understanding?
The popular definition of 'quantity of motion' is nearly meaningless(does not differentiate). Since the quantity of motion is an objects position in space, at any given instant, relative to the previous. Stating to what quantities it is deemed proportional is equally bankrupt in showing what specific phenomenon one is referring.


Here is the entry on momentum from the Oxford English Dictionary:

momentum

(məʊˈmɛntəm)

Pl. -ta.

[a. L. mōmentum: see moment n.]

†1.1 = moment 3 b. Obs.

***1735 B. Robins Disc. Newton's Meth. Fluxions 75 Sir Isaac Newton's definition of momenta, That they are the momentaneous increments or decrements of varying quantities, may possibly be thought obscure.

†2.2 ‘Impulsive weight’ (J., s.v. Moment); force of movement. Obs.

***1740 Cheyne Regimen 109 The Particles of Mercury have the greatest Momentum and Force. ***1754–64 Smellie Midwif. I. 107 In young people the Momentum of the circulating fluid is greater than the resisting force of the Solids. ***1817 Coleridge ‘Blessed are ye’ 84 The short interruptions may be well represented as a few steps backward, that it might leap forward with an additional momentum.

†3.3 Mech. = moment n. 8 b. Obs.

***1839 Penny Cycl. XV. 311 Momentum, or Moment, of Inertia.

4. a.4.a Mech. The ‘quantity of motion’ of a moving body; the product of the mass by the velocity of a body. angular momentum (see quot. 1870).

***1699 J. Keill Exam. Refl. Th. Earth 10 According to the Laws of motion, the momentum or quantity of motion of both bodies taken together would remain the same. ***1727–41 Chambers Cycl. s.v. Moment, In comparing the motions of bodies, the ratio of their momenta is always compounded of the quantity of matter, and the celerity of the moving body. ***1870 Everett Deschanel's Nat. Philos. 75 The angular momentum of a rotating body is a name given to the product of the moment of inertia and the angular velocity. ***1882 Minchin Unipl. Kinemat. 106 The momentum of a moving particle in any direction is defined to be the product of the number of units of mass in the particle and the number of units of velocity in its component of velocity in that direction.

b.4.b Hence, in popular use, applied to the effect of inertia in the continuance of motion after the impulse has ceased; impetus gained by movement.

***1860 Tyndall Glac. i. xxvii. 216 His momentum rolled him over and over down the incline. ***1874 H. R. Reynolds John Bapt. iii. iii. 189 Every drop of the Nile or the Ganges has been‥lifted by the sunbeam to the height, the fall from which give the momentum of its onward passage to the sea.

Comb. ***1880 C. & F. Darwin Movem. Pl. 508 This momentum-like movement probably results from the accumulated effects of apogeotropism.

[...]

Experimental evidence? The Earth has been going around the Sun for 4 1/2 billion years! There's your evidence right there. If 4+ billion years isn't a long enough time for you, then you will never find an acceptable answer.

This is a great example, even better is the motion of an electron around an atom. What precisely causes/d the motion of both particles? - the big bang? Do you believe such things, for the sake of self-consistency, or...

 

[sophiecentaur]

Electrons round atoms? Whilst it's "going round", do you think it's reading about Quantum Mechanics and Schroedinger's wave equation.

 

[Drakkith]

Could you reduce the difference between inertia and momentum to the perceptual level of understanding?
The popular definition of 'quantity of motion' is nearly meaningless(does not differentiate). Since the quantity of motion is an objects position in space, at any given instant, relative to the previous. Stating to what quantities it is deemed proportional is equally bankrupt in showing what specific phenomenon one is referring.

I don't know what you mean by all of this. "Perceptual level of understanding"? What exactly are you asking for?

This is a great example, even better is the motion of an electron around an atom. What precisely causes/d the motion of both particles? - the big bang? Do you believe such things, for the sake of self-consistency, or...

There is no CURRENT cause of motion other than the fact that the Earth was formed from collapsing material that already had rotation from the collapse. The source of that was, in the end, the Big Bang yes.

As to WHY I believe this, it is because current evidence supports maintsream science.

 

[Kiril]

Electrons round atoms? Whilst it's "going round", do you think it's reading about Quantum Mechanics and Schroedinger's wave equation.

No it wouldn't, at best, probabilistic approximation can tell it only that it moves(yes, in orbital configurations), not how to move(how to generate its motion).

I don't know what you mean by all of this. "Perceptual level of understanding"? What exactly are you asking for?

What I did in the opening post of this thread and what I explained in the "what is energy" thread: I'm asking for the referents of the concept, where those referents are specific entities, their relationships(causal, spacial, temporal, etc) and perceivable attributes.
And where those referents are not equivocal in their description of the concept.
"Quantity of motion" and p=mv, are not such explanations - do not lead to understanding; do not ground the concept in reality.

There is no CURRENT cause of motion other than the fact that the Earth was formed from collapsing material that already had rotation from the collapse. The source of that was, in the end, the Big Bang yes.

In other words, it rotates SOMEHOW. I don't see where you get the confidence to tell me that I'm somehow deluded about the evidence of Newtons first law.

As to WHY I believe this, it is because current evidence supports maintsream science.

In other words, because it is the most popular view. You shouldn't confuse evidence, with interpretation and inference from it; and reality with the consensus that builds around one such interpretation.

Something which proved this fact to me; the following quote is from Lanzcos' "The Variational Principles of Mechanics":

1.The variational approach to mechanics. Ever since New-
ton laid the solid foundation of dynamics by formulating the laws
of motion,the science of mechanics developed along two main
lines.One branch,which we shall call “vectorial mechanics,”
starts directly from Newton’s laws of motion.It aims at recog-
nizing all the forces acting on any given particle, its motion being
uniquely determined by the known forces acting on it at every
instant.The analysis and synthesis of forces and moments is
thus the basic concern of vectorial mechanics.
While in Newton’s mechanics the action of a force is measured
by the momentum produced by that force the great philosopher
and universalist Leibniz a contemporary of Newton advocated
another quantity, the vis viva (living force), as the proper gauge
for the dynamical action of a force. This vis viva of Leibniz
coincides, apart from the unessential factor 2 -with the quan-
tity we call today “kinetic energy.” Thus Leibniz replaced the
"momentum” of Newton by the“kinetic energy.” At the same
time he replaced the “force” of Newton by the "work of the
force.” This "work of the force” was later replaced by a still
more basic quantity, the "work function.” Leibniz is thus the
originator of that second branch of mechanics, usually called
"analytical mechanics”,which bases the entire study of equili-
brium and motion on two fundamental scalar quantities,the
“kinetic energy”and the "work function,”the latter frequently
replaceable by the“potential energy.”
Since motion is by its very nature a directed phenomenon,it
seems puzzling that two scalar quantities should be sufficient to
determine the motion.The energy theorem,which states that
the sum of the kinetic and potential energies remains unchanged
during the motion,yields only one equation,while the motion of
a single particle in space requires three equations;in the case of
mechanical systems composed of two or more particles the dis-
crepancy becomes even greater.And yet it is a fact that these
two fundamental scalars contain the complete dynamics of even
the most complicated material system,provided they are used
as the basis of a principle rather than of an equation.

[Later Lanzcos writes]

The vectorial and the variational
theories of mechanics are two different mathematical descriptions
of the same realm of natural phenomena.Newton’s theory bases
everything on two fundamental vectors:"momentum”and
“force”;the variational theory，founded by Euler and Lagrange，
bases everything on two scalar quantities:“kinetic energy”and
"work function.”Apart from mathematical expediency，the
question as to the equivalence of these two theories can be raised.
In the case of free particles，i.e.particles whose motion i s not
restricted一by given "constraints，”the two forms of description
lead to equivalent results.But for systems with constraints the
analytical treatment is simpler and more economical.

What I wish to show specifically, by these quotes, is that the terms we use(which seem otherwise so matter of fact), even in physics, represent specific interpretations(which necessarily holds the possibility of error) of reality, sometimes with the primary aim of internal theoretical consistency and then an accurate identification of physical reality. For this reason I have taken the attitude(and I suggest it to you), that my primary goal in learning physics, should be to understand the extent to which such concepts as momentum/work/energy refer to things in reality or things in themselves or are approximation(incomplete in their description).

 

[russ_watters]

Could you reduce the difference between inertia and momentum to the perceptual level of understanding?

Inertia is a property of matter, coming from mass. Momentum is mass times velocity. mv =/ m

Or to say it another way: in classical mechanics, going faster doesn't have any effect on an object's ability to accelerate.

What I did in the opening post of this thread and what I explained in the "what is energy" thread: I'm asking for the referents of the concept, where those referents are specific entities, their relationships(causal, spacial, temporal, etc) and perceivable attributes.
And where those referents are not equivocal in their description of the concept.
"Quantity of motion" and p=mv, are not such explanations - do not lead to understanding; do not ground the concept in reality.

If p=mv is insufficient for a definition of momentum, then your concept of reality is incompatible with science, so I fear you will never be able to accept scientific answers to your questions.

 

[Drakkith]

Kiril do you really think that any of us are saying that current science is all there is to know? YOU don't understand that the view you are supporting is already in place. You also don't understand that asking about something that is beyond current science to observe or reasonably theorize isn't going to get you any meaningful answers. As such, it is pointless to even discuss it if you want to accomplish anything other than to simply have a philosophical debate.

"Quantity of motion" and p=mv, are not such explanations - do not lead to understanding; do not ground the concept in reality.

Yes they do. They describe exactly how we observe the interaction of objects and forces.

In other words, because it is the most popular view. You shouldn't confuse evidence, with interpretation and inference from it; and reality with the consensus that builds around one such interpretation.

You don't even understand how those concepts fit together. I believe mainstream science BECAUSE of the overwhelming number of experiments and observations in addition to the billions of devices, machines, and other products of science that have resulted from our understanding of how the world works.

Believing that an object does NOT require a force to continue in motion is a direct result of observations. We have NEVER observed an object to slow down or accelerate without the application of a known force. This is NOT the same as saying "it will never happen" or that "it is the only explanation, ever" which you seem to be taking from this thread.

What I wish to show specifically, by these quotes, is that the terms we use(which seem otherwise so matter of fact), even in physics, represent specific interpretations(which necessarily holds the possibility of error) of reality, sometimes with the primary aim of internal theoretical consistence and then an accurate identification of physical reality. For this reason I have taken the attitude(and I suggest it to you), that my primary goal in learning physics, should be to understand the extent to which such concepts as momentum/work/energy refer to things in reality or things in themselves/are approximation.

If that is your aim, you have done a terrible job of explaining it. I will 100% agree that there are many terms in science that are redundant, or outdated, or whatever. However, your method of going about your goal does nothing but make you look like you are arguing and misunderstanding things.

 

[Kiril]

Inertia is a property of matter, coming from mass. Momentum is mass times velocity. mv =/ m

Or to say it another way: in classical mechanics, going faster doesn't have any effect on an object's ability to accelerate. If p=mv is insufficient for a definition of momentum, then your concept of reality is incompatible with science, so I fear you will never be able to accept scientific answers to your questions.

Do you understand the epistemological connection between concept and concrete?
Given your sincerity in this belief, I'm very concerned if you're a physics teacher.

 

[Drakkith]

Do you understand the epistemological connection between concept and concrete?
Given your sincerity in this belief, I'm very concerned if you're a physics teacher.

This isn't a forum for philosophy, but for mainstream science. It looks to me like you would prefer to debate the former instead of the latter.

 

[russ_watters]

Question asked and answered. Pointless to continue. Locked.