# Newton's Third Law

by protonman
Tags: newton
P: 1,509
 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?
P: 278
 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.
Emeritus
PF Gold
P: 5,532
 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.
P: 278
 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.
P: 1,509
 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?
P: 278
 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.
Emeritus
PF Gold
P: 5,532
 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.
P: 278
 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.
P: 278
 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.
P: 278
 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.
Emeritus
PF Gold
P: 5,532
 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.
 Emeritus Sci Advisor PF Gold P: 5,532 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.
 P: 618 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.
 Sci Advisor P: 674 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.
 P: 1,322 There is probably a better example of Newton's Third Law being violated by classical particles. Consider two protons. One is travelling North, the other travelling West. Both travel along the same plane. At some point their paths will cross. Let us suppose that Proton A reaches the crossing point first. When Proton B reaches the crossing point, it will exert a magnetic force on Proton A. But Proton A will not exert a force on Proton B, because Proton B is positioned along the path Proton A is travellling. Of course, the problem here is the time delay in which the electromagnetic force propagates. This problem is similar to the whirled string problem, I suppose.

 Related Discussions Introductory Physics Homework 1 Introductory Physics Homework 6 Introductory Physics Homework 3 Introductory Physics Homework 5 Introductory Physics Homework 3