# Is Newton's third law of motion wrong? Could the other ones be?

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Is Newton's third law of motion wrong? Could the other ones be?

Perhaps the original energy doesn't disappear when something impacts something else. Perhaps extra energy is gained and then dissipated as heat.

This would be when the two objects that hit each other are different shapes or maybe different sizes or maybe when they are of different springiness/hardness.

I thought this one up or realized it about a month or two ago, but forgot, so I am posting it now.

Examples of scenarios:

two magnets/electromagnets

Foot hitting pavement or arm hitting a couch or bed frame makes the secondary object really warm, but walking or running for 30 entire minutes is only supposed to be about 140 calories.

Anything hitting a wall.

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Is Newton's third law of motion wrong?

Newton's laws are not wrong, but there are scenarios for which they are not valid.

If there are fields about, such as the electromagnetic field, then that can carry away energy and momentum. Thus as far as the two bodies are concerned we get a violation of the third law. Also one has to be much more careful in quantum mechanics, where you should apply Ehrenfest's theorem.

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Newton's laws are not wrong, but there are scenarios for which they are not valid.

If there are fields about, such as the electromagnetic field, then that can carry away energy and momentum. Thus as far as the two bodies are concerned we get a violation of the third law. Also one has to be much more careful in quantum mechanics, where you should apply Ehrenfest's theorem.

Interesting, but I think that Newton might be wrong on more normal scales, too.

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Interesting, but I think that Newton might be wrong on more normal scales, too.

It seems well tested.

You have to take everything into account carefully. Real collisions are not perfectly elastic so you get some loss of kinetic energy, but we still have momentum conservation which is essentially what the third law is telling us here.

Also, you will not get momentum conservation if you have external forces applying. You need to keep track of everything carefully.

Edited by ajb
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What do you understand Newton's third Law to state?

Is it a law of motion or statics or both?

How does this lead on to a discussion of energy?

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Interesting, but I think that Newton might be wrong on more normal scales, too.

Hundreds of years of physics experiments indicates that it is not Newton that's incorrect. But feel free to explain your reasoning.

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As with all such questions, yes, it could be wrong. Science is provisional. But the evidence is such that it is exceedingly unlikely. In the realm where Newton's laws apply, they work.

I will echo what has been said before: the third law is tied to momentum conservation, not energy. Momentum conservation is also tied to linear translation symmetry — for momentum conservation to fail, the laws of physics have to vary from one place to another.

Interesting, but I think that Newton might be wrong on more normal scales, too.

What you think is of little consequence. The real issue is what you can show, i.e. what evidence do you have.

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I usually like questioning things, but when it's done from willful ignorance and misunderstanding, refusing to actually study mainstream explanations before pronouncing them WRONG, it becomes clear that this is NOT a healthy curiosity. Refutation works best when you have at least reasonable knowledge and more to go on than feelings.

In this case, I think a call to a leg manufacturer is in order. There's nothing to stand on.

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Have there been experiments for these claims in the given situations, or are we just guessing?

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Have there been experiments for these claims in the given situations, or are we just guessing?

There are plenty of simple experiments that back up Newtonian physics. I remember in high school using an air track to study collisions.

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Is Newton's third law of motion wrong? Could the other ones be?

Perhaps the original energy doesn't disappear when something impacts something else. Perhaps extra energy is gained and then dissipated as heat.

This would be when the two objects that hit each other are different shapes or maybe different sizes or maybe when they are of different springiness/hardness.

I thought this one up or realized it about a month or two ago, but forgot, so I am posting it now.

Examples of scenarios:

two magnets/electromagnets

Foot hitting pavement or arm hitting a couch or bed frame makes the secondary object really warm, but walking or running for 30 entire minutes is only supposed to be about 140 calories.

Anything hitting a wall.

Also triangles hitting each other (non-symmetric orientation ,and they may have rubber on the opposite side.

There are plenty of simple experiments that back up Newtonian physics. I remember in high school using an air track to study collisions.

Mine was only done with a computer simulation in high school.

Also triangles hitting each other (non-symmetric orientation ,and they may have rubber on the opposite side.

Mine was only done with a computer simulation in high school.

Also, you have to track the heat during these collisions, and have different shapes, sizes, and masses.

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I do believe I asked you a civil question in a non derogatory way, unlike some in this thread.

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What do you understand Newton's third Law to state?

Is it a law of motion or statics or both?

How does this lead on to a discussion of energy?

Newton's third law of motion to me means a balance of energy (Swansont mentioned momentum).

Foot hitting pavement or arm hitting a couch or bed frame makes the secondary object really warm, but walking or running for 30 entire minutes is only supposed to be about 140 calories.

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Newton's third law of motion to me means a balance of energy (Swansont mentioned momentum).

It really is momentum conservation, though not stated in quite that way usually.

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OK I will ask the question again.

studiot, on 06 Sept 2013 - 08:50 AM, said:

What do you understand Newton's third Law to state?

Is it a law of motion or statics or both?

How does this lead on to a discussion of energy?

I asked what you understand is states. Or if you will, "state Newton's third Law".

Then we can examine your proposition that it involves energy, which I don't agree with.

I don't think the man ever mentioned energy in any of his three laws.

Edited by studiot
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OK I will ask the question again.

studiot, on 06 Sept 2013 - 08:50 AM, said:

I asked what you understand is states. Or if you will, "state Newton's third Law".

Then we can examine your proposition that it involves energy, which I don't agree with.

I don't think the man ever mentioned energy in any of his three laws.

The energy is secondary and is supposed to follow along with the momentum conservation and balance of forces explanation that Swansont gave, right?

It seems well tested.

You have to take everything into account carefully. Real collisions are not perfectly elastic so you get some loss of kinetic energy, but we still have momentum conservation which is essentially what the third law is telling us here.

Also, you will not get momentum conservation if you have external forces applying. You need to keep track of everything carefully.

"Also, you will not get momentum conservation if you have external forces applying."

Does that mean I'm right?

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The energy is secondary and is supposed to follow along with the momentum conservation and balance of forces explanation that Swansont gave, right?

"Also, you will not get momentum conservation if you have external forces applying."

Does that mean I'm right?

Can you just write newtons third law as you understand it in your next post, please so we are all on the same page.

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Can you just write newtons third law as you understand it in your next post, please so we are all on the same page.

Every energy/momentum/force is supposed to create an equal and opposite energy/momentum/force when a collision occurs.

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Does that mean I'm right?

It means you have to be carefully applying the laws of physics and make sure you apply them to situations you expect them to hold.

You have to make sure the system you are studying is closed, that is no exchange of matter between the system and the outside nor any external forces. Any time we see a violation of Newton's third law, which is essentially the conservation of momentum, we quickly realise we are not actually studying a closed system.

For example, if we have charged particles in an electromagnetic field, then the electromagnetic field can carry momentum and if we do not take this into account it will look like momentum is not conserved. We will have a violation of Newton's laws, but not a violation of the conservation of momentum if we take care with the field.

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Every energy/momentum/force is supposed to create an equal and opposite energy/momentum/force when a collision occurs.

What is an energy/momentum/force please?

Which one do you mean, Energy, Momentum, Force?

Any what do collisions have to do with it, I don't recall Newton requiring any collisions, which is the point of this question in my first post

Is it a law of motion or statics or both?

In other words do you consider a block sat motionless on a table to be subject to Newton's Third Law or not?

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Momentum always comes from somewhere and goes somewhere. It never simply appears or disappears. However, if you are studying the collision of two objects and don't control everything else, there are places that momentum can enter or leave the collision other than the two objects in question.

If you take whatever else is having an effect on the system into account, momentum will be conserved.

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I usually like questioning things, but when it's done from willful ignorance and misunderstanding, refusing to actually study mainstream explanations before pronouncing them WRONG, it becomes clear that this is NOT a healthy curiosity. Refutation works best when you have at least reasonable knowledge and more to go on than feelings.

In this case, I think a call to a leg manufacturer is in order. There's nothing to stand on.

I have studied them.

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I have studied them.

Then you will appreciate the sheer elegance of what Newton said and the way he phrased his three laws.

None are redundant or wasteful, all three are necessary and together they can be used to build up the laws of classical mechanics.

Many have offered modernisations of the wording over the years, pretty well every such attempt has actually missed something in the 'upgrade'.

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Then you will appreciate the sheer elegance of what Newton said and the way he phrased his three laws.

None are redundant or wasteful, all three are necessary and together they can be used to build up the laws of classical mechanics.

Many have offered modernisations of the wording over the years, pretty well every such attempt has actually missed something in the 'upgrade'.

I hope the documentary, school, and college versions weren't different than the original.

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Does that mean I'm right?

No, It rather suggests that you don't understand what you are talking about.

It's a bit like saying

"I heard on the radio this morning that there are 1.5 dollars to the pound.

Does that mean I can sell 150 Zimbabwe dollars for £100?"

The answer is no: you are talking about different things. The US dollar exchange rate doesn't apply to the Zim dollar.

Now, as you have been asked before:

"Can you just write newtons third law as you understand it in your next post, please so we are all on the same page. "

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