Newton's Third Law equal and opposite forces

[protonman]

Although the equal and opposite forces stated in Newton's third law are sometimes referred to as an action-reaction pair in reality they do not share a cause and effect relationship. They arise simultaneously, abide together and disappear simultaneously.

Imagine you are spinning a washer attached to the end of a string in a circle. The force the string is applying to the washer is equal and opposite to the force the washer is applying to the string, which are the centripetal and centrifugal forces respectively.

The action-reaction pain as stated in Newton's third law does not share a cause and effect relationship then as soon as the string is cut both forces no longer exist.

According to relativity no signal can travel faster than light. But it the two forces cease to exist simultaneously the information that the string has been cut is instantaneously transmitted to the washer. So this information travels faster than the speed of light

 

[quantumdude]

Originally posted by protonman
According to relativity no signal can travel faster than light. But it the two forces cease to exist simultaneously the information that the string has been cut is instantaneously transmitted to the washer. So this information travels faster than the speed of light [/B]

The conclusion in this paragraph is made under the assumption that the tension in the string drops to zero at all points in the string simultaneously[/color].

In other words, the argument boils down to, "The information travels faster than light because the information travels infinitely fast."

Where is the evidence to back that up?

 

[protonman]

Originally posted by Tom
The conclusion in this paragraph is made under the assumption that the tension in the string drops to zero at all points in the string simultaneously[/color].

In other words, the argument boils down to, "The information travels faster than light because the information travels infinitely fast."

Where is the evidence to back that up?

Because the action-reaction pair stated in Newton's third law does not actually have a cause and effect relationship. They are produced, abide and destroyed simultaneously.

 

[russ_watters]

Newton's laws are classical and therefore limited by the fact that they don't take into account Relativity. The example you cite also doesn't take into account the speed of sound - not a limitation of Newton's laws, but a flaw in the model.

The limitations of Newton's laws are quite well known to physicists. So what exactly is the problem here?

 

[Janitor]

Your ideal experiment probably assumes a perfectly inelastic string. Real-world strings are elastic, and a short while after the cut has been made, the washer will still be feeling the string tension. It will take a finite time for any given part of the string to relax, and the speed at which the relaxation propagates along the string will be very much less than the speed of light.

 

[quantumdude]

Originally posted by protonman
Because the action-reaction pair stated in Newton's third law does not actually have a cause and effect relationship. They are produced, abide and destroyed simultaneously.

And that would be fine if Newton's laws held absolutely in the real universe. But they don't. In the real, physical universe, the information is not propagated along the string instantaneously.

 

[protonman]

Originally posted by Tom
And that would be fine if Newton's laws held absolutely in the real universe. But they don't. In the real, physical universe, the information is not propagated along the string instantaneously.

But you are using the thing I am questioning as you reason which is wrong. What I am questioning is the very idea that information can not be propagated instantaneously.

Anyway, are you saying that Newton's third law is does not hold absolutely in the real universe?

 

[protonman]

Ok let me clarify the point. The action-reaction pair stated in Newton's third law is not really an action-reaction pair because there is no cause and effect relationship. Therefore, they arise simultaneously, abide simultaneously and are destroyed simultaneously.

If this is the case then when I am pushing a stone with my finger the force applied by my finger and the equal and opposite force applied by the stone on my finger arise at the same time. How does the stone know that a force is being applied to it and to push back with an equal force? The information that my finger is pushing on the stone must be instantaneous because, and this is the most important point, the two forces do not share a cause and effect relationship.

 

[russ_watters]

Originally posted by protonman
Anyway, are you saying that Newton's third law is does not hold absolutely in the real universe?

Yes.

If this is the case then when I am pushing a stone with my finger the force applied by my finger and the equal and opposite force applied by the stone on my finger arise at the same time. How does the stone know that a force is being applied to it and to push back with an equal force? The information that my finger is pushing on the stone must be instantaneous because, and this is the most important point, the two forces do not share a cause and effect relationship.

Forces transfer at the speed of sound. When you start pushing on a stone, a pressure wave travels through the stone at the speed of sound (in the stone) to satisfy Newton's 3rd.

Have you heard of the "water hammer" effect? When you close a water valve quickly, the water right next to the valve stops immediatly, but the water further away does not. The water further away slams into the stopped water, stopping itself. Water even further away does the same thing, etc, etc, etc. This sends a shock wave backwards through the pipe away from the valve at the speed of sound, making a banging noise in the pipe.

When you cut a string pulling on something (in your first example), that information travels in a pressure wave through the string at the speed of sound in that string. When the pressure wave gets to the washer, then the washer "knows" the string is broken.

There IS a cause-effect relationship between the forces, but its not instantaneous and its not absolute for Newton's 3rd, at least in the way you are framing it.

 

[Michael D. Sewell]

Good analogy russ. Rapid closure of the valve results in a localized area of high pressure which compresses the water, and stretches the walls of the pipe(Hooke's law), converting the kinetic energy into elastic energy. This propagates a wave of high pressure upstream at the speed of sound. The momentum in the upstream direction causes a localized area of low pressure at the valve, causing the pipe walls to contract, starting the cycle over again(harmonic motion). Below the valve, the inverse condition causes an area of low pressure to develop, creating the same harmonic motion downstream as well. Internal and external non-conservative forces eventually dampen and end the cycle.

oops! mispelt hook. fikst nowe.

 

[quantumdude]

Originally posted by protonman
But you are using the thing I am questioning as you reason which is wrong. What I am questioning is the very idea that information can not be propagated instantaneously.

The atoms in the string are held together by electromagnetic forces. It is a well known fact that the EM force does not propagate instantaneously.

Anyway, are you saying that Newton's third law is does not hold absolutely in the real universe?

Of course. It is a fact that they don't

 

[LURCH]

Your reasoning is valid, it is only the observation you have mistaken. The thing is, this two-part statement:

Originally posted by protonman
Ok let me clarify the point. The action-reaction pair stated in Newton's third law is not really an action-reaction pair because there is no cause and effect relationship. Therefore, they arise simultaneously, abide simultaneously and are destroyed simultaneously.

If the second statement were true, it would provide a fairly good support for the first. However, the sceond statement is not true. So the fact that these interactions are not simultaneous (which is verified by all observations), actually serves as disproof of the premise that they are not cause-and-effect.

 

[protonman]

Originally posted by Tom
The atoms in the string are held together by electromagnetic forces. It is a well known fact that the EM force does not propagate instantaneously.



Of course. It is a fact that they don't

So then momentum conservation does not hold in all cases.

 

[protonman]

So when two particles touch the forces do not arise simultaneously at the mutual point of contact?

 

[russ_watters]

Originally posted by protonman
So when two particles touch the forces do not arise simultaneously at the mutual point of contact?

Yes, they do. Thats a different issue altogether.

It differs from your string-washer thought experiment in that the point of contact is where the string broke, not at the washer and your hand. At the point the string breaks, the loss of force is instantaneous for both pieces of string where they broke. But at the instant the string breaks, the washer and your hand still feel the force as if the string was intact.

 

[protonman]

Originally posted by russ_watters
Yes, they do. Thats a different issue altogether.

It differs from your string-washer thought experiment in that the point of contact is where the string broke, not at the washer and your hand. At the point the string breaks, the loss of force is instantaneous for both pieces of string where they broke. But at the instant the string breaks, the washer and your hand still feel the force as if the string was intact.

Same idea. The knowledge that the particles have hit is instantaneously transmitted to each of them.

 

[russ_watters]

Originally posted by protonman
Same idea. The knowledge that the particles have hit is instantaneously transmitted to each of them.

I honestly don't know what you're getting at. Could you elaborate?

 

[protonman]

Originally posted by russ_watters
I honestly don't know what you're getting at. Could you elaborate?

The two forces rise simultaneously at the point of contact. Since the particles do not share a cause and effect relationship what is the cause of the force on each one?

 

[russ_watters]

Originally posted by protonman
The two forces rise simultaneously at the point of contact. Since the particles do not share a cause and effect relationship what is the cause of the force on each one?

The particles do share a cause-effect relationship where the forces are concerned and Newton's 3rd applies for two particles in contact with each other.

 

[protonman]

Originally posted by russ_watters
The particles do share a cause-effect relationship where the forces are concerned and Newton's 3rd applies for two particles in contact with each other.

Which particle produces the force?

 

[quantumdude]

Originally posted by protonman
So then momentum conservation does not hold in all cases.

Yes, it does. Momentum conservation does not hang on Newton's laws, it follows from the much more general principle of the invariance of a physical system under translations in space.

edit: fixed an error

 

[quantumdude]

Originally posted by protonman
Same idea. The knowledge that the particles have hit is instantaneously transmitted to each of them.

No, it is not the "same idea".

Instead of dragging this out for pages without getting anywhere, why don't you re-read Russ' lucid post on the water in the pipe, as well as his explanation of the difference between two particles colliding and two objects connected by a string?

When the cord is cut, the information is communicated at that instant to the two segments of the string at the location of the cut[/color]. Your contention that that same information is communicated to the washer and to the holder of the string at that same instant is simply wrong.

 

[Zero]

I seem to see a problem that is being missed. There is no cause-and-effect between the two ends of the string.

 

[quantumdude]

Originally posted by Zero
I seem to see a problem that is being missed. There is no cause-and-effect between the two ends of the string.

Why not? If I jerk on one end of the string, and a short time later the washer makes a sudden move inwards, those two events are causally related. Similarly, if the string is cut, and a short time later the tension drops to zero at the end, the later is certainly caused by the former.

 

[Zero]

Originally posted by Tom
Why not? If I jerk on one end of the string, and a short time later the washer makes a sudden move inwards, those two events are causally related. Similarly, if the string is cut, and a short time later the tension drops to zero at the end, the later is certainly caused by the former.

LOL, because when you cut the string, the two ends are not acting against each other, each end is acting against the knife or scissors or whatever is cutting the string, right?

Protonman, it seems, doesn't know to factor in all the forces involved, including the forces holding the individual atoms of the string together, or the force of the scissors acting on particles at the ends of teh cut string.

 

[protonman]

Forget all the other stuff about the washer and the string because you are all missing the point.

If the equal and opposite forces between two particles do share a cause and effect relationship [as russ said] which force is the cause of the other?

 

[quantumdude]

Originally posted by Zero
LOL, because when you cut the string, the two ends are not acting against each other, each end is acting against the knife or scissors or whatever is cutting the string, right?

Right, events at one end of the string do not directly influence the state of things at the other end. That's what we've all been going around about. Let's see how much longer it goes!

 

[protonman]

Originally posted by Tom
Yes, it does. Momentum conservation does not hang on Newton's laws, it follows from the much more general principle of the invariance of a physical system under translations in space.

edit: fixed an error

I don't believe this. Momentum conservation [at least in the mechanical sense] depends on the validity of Newton's third law.

 

[quantumdude]

Originally posted by protonman
If the equal and opposite forces between two particles do share a cause and effect relationship [as russ said] which force is the cause of the other?

Ultimately, the force on each object is electromagnetic in nature (neglecting the miniscule effect due to gravity). EM forces are transmitted though local (and therefore, causal) quantum fields, mediated to first order by a virtual photon.

One force does not "cause" the other. Each force is caused by the exchange of virtual photons which transmit momentum to each of the two objects.

 

[protonman]

Originally posted by Tom
Ultimately, the force on each object is electromagnetic in nature (neglecting the miniscule effect due to gravity). EM forces are transmitted though local (and therefore, causal) quantum fields, mediated to first order by a virtual photon.

One force does not "cause" the other. Each force is caused by the exchange of virtual photons which transmit momentum to each of the two objects.

You need to address the question in everyday terms. I don't accept virtual particles or quantum fields or any of that. In reality, although QM may make excellent predictions it is wrong from an ontological point of view.

But let's not get off the topic. Russ said there is a cause and effect relationship, you say there is not. Well which one is it?

 

[quantumdude]

Originally posted by protonman
I don't believe this. Momentum conservation [at least in the mechanical sense] depends on the validity of Newton's third law.

No, it does not. Momentum conservation can be derived, via Noether's theorem, from any physical theory that can be expressed in terms of a Lagrangian. Quantum mechanics and quantum field theory can both be so expressed, and neither of them are consistent with Newton's third law.

 

[quantumdude]

Originally posted by protonman
You need to address the question in everyday terms.

You need to update your everyday terms.

I don't accept virtual particles or quantum fields or any of that. In reality, although QM may make excellent predictions it is wrong from an ontological point of view.

Then you need to change your ontological point of view, because experiment is the final court of appeals on scientific matters.

But let's not get off the topic. Russ said there is a cause and effect relationship, you say there is not. Well which one is it?

Both.

Russ said there is a cause and effect relationship between the particles[/color].

I said that there is no cause and effect relationship between the forces[/color].

Sure enough, one particle would not feel any force were it not for the presence of the second particle. So, there is a causal relationship between the two objects. What I said is that one force is not caused by the other force, but rather that both forces are caused by an underlying interaction.

 

[Janitor]

Not meant to complicate the thrust of this little debate--

EM forces are transmitted though local (and therefore, causal) quantum fields, mediated to first order by a virtual photon.

This is off topic, but what you said reminded me of something that I have wondered about in the past. Is the fermion degeneracy pressure (such as in a neutron star or what have you) to be viewed as stemming from exchange of virtual bosons? I'm thinking the answer is "no" since it is always explained as being due to the rule that identical fermions cannot occupy the same state, and that they thereby stay about a wavelength or more apart from one another. Any further clarification on this from the experts would be appreciated.

 

[Zero]

Originally posted by protonman
Forget all the other stuff about the washer and the string because you are all missing the point.

If the equal and opposite forces between two particles do share a cause and effect relationship [as russ said] which force is the cause of the other?

In what situation are we talking about, if you want us to ignore your previous example?

 

[Zero]

Originally posted by protonman
You need to address the question in everyday terms.

Why? I thought you were smarter than everyone else when it came to physics? Now you are saying that you can't keep up your end of the physics conversation unless we speak in 7th-grader terms?

 

[protonman]

Originally posted by Zero
In what situation are we talking about, if you want us to ignore your previous example?

Two particles collide. At their point of contact there are equal and opposite forces exerted by each on the other. The question was do the forces share a cause and effect relationship or not.

 

[Zero]

Originally posted by protonman
Two particles collide. At their point of contact there are equal and opposite forces exerted by each on the other. The question was do the forces share a cause and effect relationship or not.

Sure they do, after the collision...what's your point? If you roll two balls against each other, after they hit each other, the path each one takes is determined by the collision with the other, which shows a causal relationship?

 

[protonman]

Originally posted by Zero
Sure they do, after the collision...what's your point? If you roll two balls against each other, after they hit each other, the path each one takes is determined by the collision with the other, which shows a causal relationship?

No you are wrong. Here is why. If particle A hits particle B and causes it to move then the force from A caused B to move. This is correct. But this says nothing about a relationship between the force A applies to B and the force B applies to A.

You are completely wrong. What I am asking about is whether there is a causual relationship between the force that A exerts on B and the force B exterts on A.

In your explanation there is no causual relation between A and B because the fact that A causes B to move has nothing to do with the fact that B applies an equal and opposite force on A. The only forces producing any kind of movement on B are forces external to B. This is physics 101.

 

[quantumdude]

Originally posted by protonman
You are completely wrong. What I am asking about is whether there is a causual relationship between the force that A exerts on B and the force B exterts on A.

I already answered that. All you did was say, "I don't accept quantum theory", which does not qualify as a rebuttal.

The only forces producing any kind of movement on B are forces external to B. This is physics 101.

Tell that to a hamster next time you see him running around in his little hamster ball.

 

[protonman]

Then you need to change your ontological point of view, because experiment is the final court of appeals on scientific matters.

I completely disagree with you and here is why. Physics is based on observation. This functions fine for the everyday level where we have direct access to experiments via our senses. When we get to the very small level though problems arise. Here we can not rely on our sense and therefore must resort to inference. Without a proper knowledge of correct and incorrect methods of inference we can not be sure that scientific experiments done on this level are valid.

Essentially, you are assuing that a theory's congruence with experiment is suffcient evidence to establish its ontological validity. There are many examples that disprove this view. The most obvious of which is the fact that [from the point of view of the physics community] SR established that Newtonian mechanics does not give accurate predictions at high velocities.

I really don't want to get off the topic but this need to be addressed.

 

[Zero]

Originally posted by protonman
No you are wrong. Here is why. If particle A hits particle B and causes it to move then the force from A caused B to move. This is correct. But this says nothing about a relationship between the force A applies to B and the force B applies to A.

You are completely wrong. What I am asking about is whether there is a causual relationship between the force that A exerts on B and the force B exterts on A.

In your explanation there is no causual relation between A and B because the fact that A causes B to move has nothing to do with the fact that B applies an equal and opposite force on A. The only forces producing any kind of movement on B are forces external to B. This is physics 101.

Huh? When A hits B, does A change speed or direction? Yes. Does B change speed or direction? Yes. Therefore, the speed and direction of A are causally linked to B, and the speed and direction of B are both causally linked to A.

Why does this basic fact of physics confuse you so much? Have you asked your mom, dad, or one of your teachers to try explaining it to you?

 

[protonman]

I already answered that. All you did was say, "I don't accept quantum theory", which does not qualify as a rebuttal.

This is a classical physics board and there is a reason why I posted this here.

It shouldn't matter and you shouldn't need to explain macroscopic things through appeal to the microscopic level.

I will be up front with you. I am a Buddhist and there are many things I could bring up that are confirmed experimentally in Buddhism. The reason I don't bring them up is that you are not a Buddhist and my introducing these ideas would be paramount to a Christian quoting the Bible. I am arguing my case purely on logical grounds. My not accepting your introduction of experimental evidence is not a cheap ploy to ignore something that refutes my points. I just don't accept it in the same way you don't accept the experimental results of my view. I think QM is wrong ontologically, although I said it is certainly valueable in so far as it can make correct experimental predictions. If two people are going to debate there needs to be a common accepted ground of terminology. Otherwise, for one, either person could simply make up stuff. Therefore, I hope that we can debate this on the macroscopic level since this post in under classical physics.

 

[quantumdude]

Originally posted by protonman
Without a proper knowledge of correct and incorrect methods of inference we can not be sure that scientific experiments done on this level are valid.

But we do have knowledge of correct and incorrect methods of inference. Scientists use the same logic that philosophers and mathematicians use.

Essentially, you are assuing that a theory's congruence with experiment is suffcient evidence to establish its ontological validity.

What I am assuming is that ontology is not of first priority when determining which scientific theory is to be accepted. This is due to the fact that ontology (which is a priori) is not compatible with the way the universe is known (which is a posteriori), so it is ill-suited to be the basis of a scientific method.

There are many examples that disprove this view. The most obvious of which is the fact that [from the point of view of the physics community] SR established that Newtonian mechanics does not give accurate predictions at high velocities.

Actually, that would work against the "ontology first" view. It is not SR that "established" that Newtonian mechanics does not work at high velocities, it is experimental evidence that "established" it. One theory cannot be used to discredit another theory.

I really don't want to get off the topic but this need to be addressed.

It's really not off topic at all, because you are asking questions that can only be properly answered by considering the nature of the forces involved. Such issues are not addressed in any way, shape, or form in Newton's laws, and we are forced to look at quantum theory. Since you don't accept quantum theory, we find ourselves here.

 

[protonman]

Huh? When A hits B, does A change speed or direction? Yes. Does B change speed or direction? Yes. Therefore, the speed and direction of A are causally linked to B, and the speed and direction of B are both causally linked to A.

This is exactly what I said.

I am asking is there a casual relationship between the forces A and B each exert on one another.

 

[protonman]

What I am assuming is that ontology is not of first priority when determining which scientific theory is to be accepted. This is due to the fact that ontology (which is a priori) is not compatible with the way the universe is known (which is a posteriori), so it is ill-suited to be the basis of a scientific method.

Maybe we have different understanding of ontology. Basically ontology is the study of what exists. This is not a priori. We can know what exists a posteriori.

In addition, there are many refutations of QM in Buddhist literature. The fact is if you can't argue this on classical grounds then you really don't understand it. The reality of the small can not negate the reality of the large.

If you are not going to argue this way then then you might as well just leave the argument to those of us who can.

 

[protonman]

But we do have knowledge of correct and incorrect methods of inference. Scientists use the same logic that philosophers and mathematicians use.

Not eastern philosophers.

 

[quantumdude]

Originally posted by protonman
This is a classical physics board and there is a reason why I posted this here.

The subforums are established for organizational convenience. They are not intended to serve as blinders.

It shouldn't matter and you shouldn't need to explain macroscopic things through appeal to the microscopic level.

Actually, we can only explain certain things about the macroscopic world through appeal to the microscopic level. As I just explained in my last post, classical physics does not have one thing to say about the nature of forces. It only describes their effects.

I am arguing my case purely on logical grounds.

But you are not arguing on "purely logical grounds". If you were, then your posts would look something like:

[p-->q]^(~q)-->~p

or some such like.

It is not possble to argue a case based on "pure logic". Premises have to be chosen to fill in those logical variables, and those premises are bound to be reflective of your worldview.

Therefore, I hope that we can debate this on the macroscopic level since this post in under classical physics.

We can't, since you are arguing within the framework of a falsified, acausal theory.

 

[quantumdude]

Because this thread has gotten so far off topic, I have split off the philosophical discussion into a thread called Ontology and Logic, located in the Metaphysics and Epistemology Forum.

 

[jdavel]

Zero posted:

"When A hits B, does A change speed or direction? Yes. Does B change speed or direction? Yes. Therefore, the speed and direction of A are causally linked to B, and the speed and direction of B are both causally linked to A."


What if A & B are charged particles that interact only through their coulomb fields? Suppose that A is suddenly and breifly pushed toward B by some third force. A immediately experiences a new force from B. But B won't experience any change in force for a time x/c where is x is the distance between A & B, and c is the speed of light. It seems that in this case, the force that B exerts on A could be causing the force that A exerts on B, since the B on A force happens first.

And during the time interval x/c, the total momentum of A & B is not conserved. Newton didn't know about charged particles and the fact that they're interaction travels at finite speed. But he knew a bit about massive particles that interact through gravity. He defended the absolute validity of his 3rd law by assuming the speed of the gravity interaction was infinite.

So the true "classical" answer to this question of whether the force that one of two interacting particles exerts on the other causes the force that the other exerts seems to be no. The interaction appears at the same instant at each one, so neither one could cause the other.

The real truth, however, seems to be that the force on the one that moves first is the causer.

 

[Stingray]

You mentioned a while back the connection between the 3rd law and momentum conservation. You're right that mechanical momentum conservation requires it, but there are no known forces which actually enforce mechanical momentum conservation!

There are explicit examples in textbooks showing that electromagnetism does not always obey the 3rd law for example.

Of course we still like to say that momentum is conserved, but we define it more generally (using the translation invariance that was mentioned above). When you do that, you obtain that the fields themselves carry momentum (and energy). So the sum of the field momentum+mechanical momentum is conserved, but not the mechanical momentum alone.