Is Newton's 3rd law incorrect?

[pradipta]

is the concept hypthetical

 

[Abullais Ghazi]

No it's not incorrect...It is applicable for macroscopic bodies but not applicable for microscopic bodies.

 

[DEvens]

No it's not incorrect...It is applicable for macroscopic bodies but not applicable for microscopic bodies.

Microscopic as well.
https://en.wikipedia.org/wiki/Atomic_recoil

 

[Suraj Powar]

To every actions there's equal and opposite reaction is not hypothetical.
It is true for microscopic and macroscopic bodies as the sizes of forces acting on bodies are acting equally.
But we can't say that the reaction is instantaneous as when a chemical reaction takes place the reaction doesn't take place in seconds it does take time.

 

[DEvens]

To every actions there's equal and opposite reaction is not hypothetical.
It is true for microscopic and macroscopic bodies as the sizes of forces acting on bodies are acting equally.
But we can't say that the reaction is instantaneous as when a chemical reaction takes place the reaction doesn't take place in seconds it does take time.

Chemical reactions are not what is being referred to in Newton's third law.

 

[Nugatory]

is the concept hypthetical

No. But why are you thinking it might be?

 

[Karan Punjabi]

No. But why are you thinking it might be?

Newton's third law work for every object microscopic as well as macroscopic. And it is instantaneous just think about the fact that when someone shoots a bullet from the rifle or any gun the recoils occur at that moment only and it occurs in the opposite direction of bullet fired

 

[lightarrow]

Newton's third law work for every object microscopic as well as macroscopic. And it is instantaneous just think about the fact that when someone shoots a bullet from the rifle or any gun the recoils occur at that moment only and it occurs in the opposite direction of bullet fired

It can't be instantaneous in the sense that c is the limiting speed of interactions.

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[Karan Punjabi]

It can't be instantaneous in the sense that c is the limiting speed of interactions.

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Albert Einstein proved through mathematical equations that the carriers which carry the information of gravity calles gravitons work at the speed of light so gravity works as fast as light

 

[HallsofIvy]

So what is your point? What does the transfer of the reaction from one part of the gun to another or to your shoulder have to do with Newton's third law?

 

[willem2]

It can't be instantaneous in the sense that c is the limiting speed of interactions.

Both forces can still be the same and in opposite directions at the same time, because both forces will be delayed by the same amount.

 

[jtbell]

Consider two equal positive charges q₁ and q₂. q₁ is traveling along the x-axis in the +x direction, and and q₂ is at rest on the +y-axis, a distance y away from the origin. Calculate the Lorentz forces $\vec F_1 = q_1 (\vec E_2 + \vec v_1 \times \vec B_2)$ and $\vec F_2 = q_2 (\vec E_1 + \vec v_2 \times \vec B_1)$ at the moment that q₁ passes through the origin. Are those two forces equal in magnitude and opposite in direction?

 

[pradipta]

if i give a 10 Newton force to an object say a steel scrap then the force will transfer automatically to sound energy,heat energy etcetc and during this time there will be a loss in energy which in return may give 9.5 or 9.8 or so but it can't give exact or equal to 10 Newton...

 

[DrStupid]

It can't be instantaneous in the sense that c is the limiting speed of interactions.

It can if the interaction is locally only. Of course this requires a much more general concept of "body".

 

[lightarrow]

It can if the interaction is locally only. Of course this requires a much more general concept of "body".

Can you expand a bit?

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[nasu]

if i give a 10 Newton force to an object say a steel scrap then the force will transfer automatically to sound energy,heat energy etcetc and during this time there will be a loss in energy which in return may give 9.5 or 9.8 or so but it can't give exact or equal to 10 Newton...

This description is flawed. You don't "give" a force and then receive another one back.
The force is a measure of interaction between two objects. If there is an interaction between you and another object, you and the object simultaneously act with a force of 10N on each other. If there are other forces acting at the same time, they are part of other 3rd law pairs. They can decrease the energy of the system, maybe.

 

[DrStupid]

Can you expand a bit?

The interaction between two distant charged particles can't be instantaneous but the local interaction between the particles and their electromagnetic field is instantaneous. Newton's third law applies to this situation if photons or even virtual photons are considered to be "bodies".

 

[Chestermiller]

if i give a 10 Newton force to an object say a steel scrap then the force will transfer automatically to sound energy,heat energy etcetc and during this time there will be a loss in energy which in return may give 9.5 or 9.8 or so but it can't give exact or equal to 10 Newton...

This is not related to Newton's 3rd law. Newton's third law says that the force you exert on the steel scrap is the same as the force that the steel scrap exerts back on you.

 

[russ_watters]

This description is flawed. You don't "give" a force and then receive another one back.
The force is a measure of interaction between two objects. If there is an interaction between you and another object, you and the object simultaneously act with a force of 10N on each other. If there are other forces acting at the same time, they are part of other 3rd law pairs. They can decrease the energy of the system, maybe.

To amplify even a bit more: you don't "give" a force and then receive another one back because that implies that they are separate events. They aren't. The application of/exchange of forces is a single event.

 

[lightarrow]

The interaction between two distant charged particles can't be instantaneous but the local interaction between the particles and their electromagnetic field is instantaneous. Newton's third law applies to this situation if photons or even virtual photons are considered to be "bodies".

It seems very very "stretched" to me, considered that many physicists consider virtual photons as nothing else than a mathematical "trick".

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[ayush solanki]

Albert Einstein proved through mathematical equations that the carriers which carry the information of gravity calles gravitons work at the speed of light so gravity works as fast as light

I don't think that graviton is a proved particle yet.is it?

 

[ZapperZ]

if i give a 10 Newton force to an object say a steel scrap then the force will transfer automatically to sound energy,heat energy etcetc and during this time there will be a loss in energy which in return may give 9.5 or 9.8 or so but it can't give exact or equal to 10 Newton...

Next time, when you make a thread asking for something like this, please also include THIS as your "supporting argument" in that very first post. Otherwise, your question just hangs there without any kind of justification or impetus on why you would ask such a thing (did it come to you in a dream?).

Furthermore, as nasu has replied, we can also correct the mistake in your understanding on why you would think of such a thing based on this example. It is a more CONCRETE and clearer way to address such a mistake, rather than some hand-waving description on why your understanding is faulty.

As a reminder, please note that the PF Rules require that you present a clear and as complete of a post as possible when presenting a question such as this.

Zz.

 

[DrStupid]

It seems very very "stretched" to me, considered that many physicists consider virtual photons as nothing else than a mathematical "trick".

Isn't the third law a mathematical "trick" too? There is no reason why forces must cancel each other pairwise out. Conservation of momentum just requires that the sum of all forces is zero in inertial systems. For example, if there are three bodies A, B and C this would allow that

A exerts a force F_AB to B but no force to C
B exerts a force F_BC to C but no force to A
C exerts a force F_CA to A but no force to B

with

F_AB + F_BC + F_CA = 0

The third law does not allow such ternary interactions and requires to describe the same situation with three binary interactions. Due to the superposition principle this is always possible but it is impossible to distinguish the two cases experimentally. We are using the binary interactions because they appear to be useful. But we cannot show that they really exist in systems with more than two interacting bodies. In this sense they are just a mathematical "trick". But there is nothing wrong with it.

 

[pradipta]

This description is flawed. You don't "give" a force and then receive another one back.
The force is a measure of interaction between two objects. If there is an interaction between you and another object, you and the object simultaneously act with a force of 10N on each other. If there are other forces acting at the same time, they are part of other 3rd law pairs. They can decrease the energy of the system, maybe.

other forces means?specfy

 

[nasu]

I don't know, those ones you had in mind when you described dissipation of mechanical energy.

 

[lightarrow]

Isn't the third law a mathematical "trick" too? There is no reason why forces must cancel each other pairwise out. Conservation of momentum just requires that the sum of all forces is zero in inertial systems.

And what does conservation of angular momentum requires?

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[DrStupid]

And what does conservation of angular momentum requires?

That requires that the sum of all torques is zero but that's another topic.

 

[lightarrow]

That requires that the sum of all torques is zero but that's another topic.

Why it's "another topic"? We are discussing "Newton's 3d law", not "first law of statics".

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[DrStupid]

Why it's "another topic"?.

Because we are discussion Newton's 3rd law and not conservation of angular momentum.

 

[lightarrow]

Because we are discussion Newton's 3rd law and not conservation of angular momentum.

And why do you think that conservation of angular momentum is not related to Newton's 3d law?

Edit. I explain.
For a system of mass points, the fact the resultant moment of internal forces is zero comes from Newton's 3d law. Conservation of the system's angular momentum can be proved using this fact.

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[jbriggs444]

And why do you think that conservation of angular momentum is not related to Newton's 3d law?

Edit. I explain.
For a system of mass points, the fact the resultant moment of internal forces is zero comes from Newton's 3d law. Conservation of the system's angular momentum can be proved using this fact.

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

If you allow for force at a distance then it does not follow immediately. It requires the additional condition that forces at a distance must operate in a direction on the axis between the point particles upon which they act. That additional condition must be met if we demand that the laws of nature be isotropic with respect to direction.

 

[lightarrow]

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

If you allow for force at a distance then it does not follow immediately. It requires the additional condition that forces at a distance must operate in a direction on the axis between the point particles upon which they act. That additional condition must be met if we demand that the laws of nature be isotropic with respect to direction.

Certainly. I should have specified that the law of conservation of angular momentum requires that additional condition, thank you to have noticed.

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[DrStupid]

And why do you think that conservation of angular momentum is not related to Newton's 3d law?

The 3rd law conserves momentum but not angular momentum. That would be the case if the forces between two bodies always act parallel to their displacement vector (as jbriggs444 already told you) but Newton's laws of motion do not include such a condition. In fact they do not say anything about angular momentum. Momentum and angular momentum are completely different things.

For a system of mass points, the fact the resultant moment of internal forces is zero comes from Newton's 3d law. Conservation of the system's angular momentum can be proved using this fact.

The fact that the 3rd law can be used to prove something else does not mean that something else is the same topic.

 

[DrStupid]

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

This still needs the additional assumption that changes of angular momentum always require forces and moment arms. Does that apply to spin-transfer?

 

[ogg]

Elementary blunder #1: No definition of what is MEANT by "Newton's 3rd Law". It is generally assumed it refers to rigid particles and deals with their dynamics. Newton's 3rd Law is an expression of an underlying conservation law and as Noether proved, a conservation law requires an "equivalent" symmetry (which is an abstract concept rather than a physical "thing"). Elementary blunder #2: failure to maintain strict logical separation between an object (or set of objects) of interest and the "system". What we know: Energy and momentum are conserved locally. (by locally, I mean at distances less than those in which cosmological expansion becomes significant ~ mega-lightyear.) Information is conserved locally. (by locally, I mean within our observable universe, which 'ends' at any and all event horizons).