# Newton's Third Law

by protonman
Tags: newton
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P: 21,658
 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.
P: 278
 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?
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P: 5,539
 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
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 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. 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.
 P: 1,509 I seem to see a problem that is being missed. There is no cause-and-effect between the two ends of the string.
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 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.
P: 1,509
 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.
 P: 278 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?
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 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!
P: 278
 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 belive this. Momentum conservation [at least in the mechanical sense] depends on the validity of Newton's third law.
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P: 5,539
 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.
P: 278
 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 lets not get off the topic. Russ said there is a cause and effect relationship, you say there is not. Well which one is it?
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P: 5,539
 Originally posted by protonman I don't belive 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.
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 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 lets 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.

I said that there is no cause and effect relationship between the forces.

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.
P: 1,182
 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.
P: 1,509
 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?
P: 1,509
 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?
P: 278
 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.

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