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Parts of the article have been flagged as needing citations. I've created this Talk Page Section so that editors can focus on that here.
Here is a complete re-write of this article. I have removed the stub pointer. Comments and suggestions are welcome. -- Michel M Verstraete 22:24, 21 June 2006 (UTC)
Dear Stephen, Thanks for your kind words, and apologies for the delayed response to your question: I only just saw your comment today.
The car crashing into a wall is subject to the same laws of physics as all other objects of the Universe, but this case is rather more complicated, because it involves not just objects moving with respect to each other but also deforming (to the point of breakdown). The simple answer is that the car will likely be destroyed too (or at least severely dented) in this experiment. If you replace the car by a cyclist, you will see that in that case the wall stands and the cyclist is hurt: it thus has to do with the strength or solidity of the objects that interact. The fact that the wall collapses does not imply it has no impact on the car.
A more detailed answer would go along the following lines: As soon as the car bumper touches the wall, it exerts a pressure force on the wall, and the wall exerts a similar (reaction) pressure force on the car. As long as these forces remain 'small enough', they only result in elastic deformations of both objects, following [Hooke's law] (Visualize a plastic bumper and a carton wall, for instance). As the pressure from the car increases, the reaction from the wall also increases. At some point, the structure of some of the materials involved will break down, the interaction becomes [Elasticity (physics)|inelastic], and the weaker one looses its ability to hold itself in one piece and thus also its ability to react. At that point, the car does not apply any force on the wall anymore (the contact point or surface has vanished) and thus the wall does not exert a force on the car either. The fact that the wall collapses is a subsequent consequence of the structural instability of the wall itself, following the piercing of a large hole and/or the shock wave induced into the wall; this has little or nothing to do with the car, as far as the action-reaction law is concerned.
I hope this helps. Michel M Verstraete 22:50, 5 June 2007 (UTC).
The article currently has a paragraph that reads:
I've never heard about this. Can we have a source for it? Hairouna 04:13, 12 September 2007 (UTC)
Think of it this way: While the definition of a force involves an accelerating mass, this thing does not normally exist in isolation. So if one billiard ball strikes another, at the moment of impact there is a force affecting BOTH balls. The simplest way to generically say it is, "A force typically comes into existence between two interacting things. It is the interaction that yields the force, and simply because both things are involved in a particular type of interaction, both things experience the same type of force."
All that said, I'm not sure I can agree with the claim that the preceding is always true for every possible kind of force. For example, an electron can absorb a photon, and acquire the kinetic energy and momentum of the photon. WHEN THIS EVENT happens, DURING it, the photon disappears and the electron experiences a force that accelerates it to a new velocity. I acknowledge this event properly belongs to the realm of Quantum Mechanics, where strange things are allowed (the electron may be described as literally instantly starting to move at the new velocity, and no Newtonian-type of acceleration/force may be involved at all). Nevertheless, this event does pose a problem with respect to the notion of Action and Reaction, simply because, while obviously the electron is Acting, after absorbing the photon, nothing exists that is Reacting! —Preceding unsigned comment added by 216.9.73.2 ( talk) 08:19, 7 January 2008 (UTC)
I've edited the explanation of one of the misunderstandings, to highlight where the confusion comes from. The force exerted by the table can be rightfully called reaction (and indeed that is often the case, as in
ground reaction), as long as it's clear what force it is a reaction to.
Also, I don't see why the table and the book are not at rest (in any inertial frame of reference). I think the statement implicitly assumes the Earth as reference frame, neglecting any effects due to its rotation, therefore both book and table are at rest, with respect to it. Anyway, the at-rest question is non-relevant to the 'weight-action/table-reaction' confusion and so I've removed it.
Giuliopp (
talk)
19:25, 17 November 2007 (UTC)
The statement "This is not the case, since the two forces are different in nature and are both applied to the book; . . ." cannot be correct. The book's weight is not applied to the book. A gravitational force is applied to the book. Weight is a force exerted by a massive object in a gravitational field (the book) on another object or surface (the table) that restrains motion toward the center of the gravitational field. No gravity, no weight. No restraint, no weight. Objects in free-fall have mass, but no weight. spottydog3 ( talk) 14:07, 30 September 2011 (UTC)
A new misunderstanding has been added. One way to test the description involves the toy Newton's cradle. It usually has about 5 suspended balls. Why do you never see a version of that toy with, say, 200 balls? (If you worry about accurate alignment, just replace all the center balls with a simple rod.) The answer relates to the time it takes the mechanical force to propagate to the last ball in the row. When the distance between first and last ball is short, the system allows essentially all the momentum to transfer easily from the first ball to the last ball. But when the distance is long, the first ball partly bounces back, and the last ball doesn't swing out as far as it does when the distance is short. The row of balls is acting like a massive solid object, NOT INSTANTLY RESPONDING AS A WHOLE TO THE APPLIED FORCE, until the last ball starts to move, and it is that "acting like a solid object" that causes the first ball to bounce. In this case, then, the action/reaction of the impact force between the first ball and the row occurs simultaneously, but the actual movement-action/reaction of the bodies is non-simultaneous, exactly as is described in the added "misunderstanding" text. —Preceding unsigned comment added by 216.9.73.101 ( talk) 23:30, 1 January 2008 (UTC)
Hi all,
I'm obviously not a math pro, the F=ma doesn't make sense to me. If there is an object at rest in space and has i.e. the mass of 10, then its force should be 0. F = 10*0
So if there are two objects placed near each other, both are at rest, what is the determining factor for their gravitational force. Mass only? Furthermore are the masses added up to form one force that acts on both objects? —Preceding unsigned comment added by 91.57.156.132 ( talk) 18:13, 12 March 2010 (UTC)
Since you have the clear idea about "A car driving in a curve exerts a centrifugal force on the road." I think you may just say, the "experiences" in the sample sentence "The centrifugal force that an object experiences is the reaction to the centripetal force on that object." could be replaced by "exerts to a rotation container". That will make it a clear statement. Jh17710 ( talk) 02:57, 9 January 2011 (UTC)
In the hammer throw example it states that "the athlete exerts an outward centrifugal force on the ball, the ball exerts an inward 'centripetal' force on the athlete". I'm not sure whether I don't understand the wording or this is wrong, but I'm pretty sure the athlete exerts the centripetal force on the hammer, preventing it from leaving the orbit in a straight trajectory. The ball pulls the athlete outwards and this pulling is counteracted by the inward pulling of the thrower. The person isn't in a position to exert a centrifugal force since centrifugal forces aren't exerted by an object, they occur in the a system due to the acceleration of the system.-- 129.247.247.238 ( talk) 08:00, 2 October 2012 (UTC)
I know that physicists can enjoy being pedantic, but there really are errors in this article, I'm sure, where someone has tried to make too big a deal out of nothing and ended up actually writing nonsense. Uncited and unreferenced nonsense. Here we introduce 'Examples of common misunderstandings' by saying "Newton's third law is frequently stated in a simplistic but incomplete or incorrect manner through statements such as [...] To every action there is an equal and opposite reaction". Yet this is the very wording that Newton's laws of motion uses, referenced to Newton's Principia, "To every action there is always an equal and opposite reaction", or in the reference, "To every action there is always opposed an equal reaction". [1]
Is the author here so clever that they are saying that Newton didn't really understand Newton's 3rd Law (but they do)? In the book on a table and the weight on a string or spring examples, a great deal of words are used to explain something terribly obscure and complicated, but later in the same reference, Newton wrote, "Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone". Here we expend a lot of hot air, but can we not just say that the book presses on the table and the table presses on the book?
If there is an understanding of Newton's 3rd Law to be had that is so subtle that it evaded even Newton, then I have to say that I have not found out what it is from all the verbosity, hot air and {citation needed} tags in this article. I enjoy proving generations of science teachers wrong as much as the next person, but I think we fall short of doing so here. Like the rest of Wikipedia, we should first find the references that explain why everyman is wrong, and then summarise the sources' arguments here - no more and no less. -- Nigelj ( talk) 14:55, 8 October 2011 (UTC)
This article focuses on misunderstandings of Newton's Third Law. As a college instructor in introductory physics, I appreciate the need to address such misunderstandings, but I doubt whether it should be done in an encyclopedia; certainly, it should not be the main substance of the main article. The main article should make a positive statement of the Third Law, together with a list of examples.
The various misunderstanding of the Third Law, with appropriate rebuttals, could be listed in a separate article.
Speaking of misunderstandings, the term 'reaction force' is used differently in physics literature. While introductory physics texts use it in the Third-Law sense, as reference to the other half of an interaction force pair, applied texts (e.g. biomechanics) mean by it the normal force by the ground on a supported weight. The current article correctly implies that the weight of the object and this normal force are not an action-reaction pair; this is easily seen from the example of an accelerated elevator, where the normal force has a different magnitude than the object's weight.
I am therefore not so sure that 'reaction force' is the best main title for this article. I would propose 'force pairs', or 'interaction pairs'. The fact that Newton himself spoke of actio/reactio does not impress me much. Surely, no one would suggest that we call inertia an 'intrinsic force' just because Newton did so in his 'Principia'...
In the article on Newton's Laws, some aspects of the 'reaction force' are addressed that deserve elaboration. For instance, Newton's Third Law is implied by the law of conservation of momentum. (If ∆p[system] = ΣF[external]t on one hand, but ∆p[system] = Σ∆p[parts] = ΣF[parts]t = ΣF[external]t + ΣF[internal]t on the other, then ΣF[internal] = 0 for any interaction.) Arjenvreugd ( talk) 01:56, 1 November 2011 (UTC)
The section currently says this:
F1. gravitational force by earth on object (downward) F2. gravitational force by object on earth (upward) F3. force by support on object (upward) F4. force by object on support (downward)
Forces F1 and F2 are equal because of Newton's Third Law; the same is true for forces F3 and F4. Forces F2 and F3 are only equal if the object is in equilibrium, and no other forces are applied. This has nothing to do with Newton's Third Law.
[end of quote]
In the penultimate sentence, should it refer to F1 and F3 being equal, rather than F2 and F3? (F2 and F3 are both acting upwards). 109.149.189.15 ( talk) 14:55, 17 October 2012 (UTC)
The statement that all forces come in pairs is a pretty common summary of Newton's third law. I don't think it's necessarily ambiguous, especially because the same sentence continues to qualify what constitute force pairs, ie if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. In the counter example provided by the editor removing the text, "a magnet pushes downward with its weight plus some magnetic component, but the earth pushes up with just one normal (electrostatic) force, that's 3 forces in balance. There's more to it of course, but "pairs" misleads", ignores the other forces in this interaction between the magnet and the earth. Going through the forces:
Note that none of the forces on the magnetic form a pair with any of the other forces on the magnet, which fulfills the second half of the "all forces come in pairs". Honestly, the sentence makes less sense to me without the "in pairs" comment - without it there is no motivation for the "such that" clause that follows. Can you explain why you see the pairs statement as ambiguous or misleading, because I'm not seeing it? What kind of can of worms does it open? -- FyzixFighter ( talk) 01:26, 28 September 2016 (UTC)
Hi FyzixFighter,
Your bulleted dissection showing that you understand supports my point perfectly. The bullets are excellent and correct. But, getting you and me to understand is not the goal here. Leading lay readers clearly (with reliable refs) is the goal.
The worst thing about "All forces come in pairs" is that it carelessly leads many lay readers to (incorrectly) conclude that there can never be a non-zero net force -- a problem I'm sure you're familiar with.
"All forces come in pairs" is frequently used by sources who don't really know what they're implying. Lot's of sources use the expression, and most of them don't use it clearly -- because they don't clarify what it doesn't mean. Many of them even mis-phrase it so it's just plain wrong.
It's way too easy to find an unreliable source that says "all forces come in pairs". Reliable sources aren't so careless as to go there. In fact, I'd go so far as to say that any source spouting a variant of "all forces come in pairs" should be considered dubious until a close careful look proves otherwise!
Your burden ( WP:burden) is to show that your sources are reliable. But even if they do explain it clearly without leaving open misinterpretations (which I truly hope they do), our text must also be clear in and of itself. Simply saying "all forces come in pairs" (even with reliable refs) doesn't achieve that. Luckily, there's an easy solution -- just leaving out "in pairs" altogether obviates the need to explain what it doesn't mean (with such long-winded explanations as the bulleted dissection and explaining that it doesn't mean forces always add to zero). "In pairs" just isn't necessary to get the point across, and there are are tons of reliable refs that don't say it.
Some thoughts for fun:
1) "forces exist in pairs" really only applies to the two directions of a force-carrying particle's transfer of momentum between the two particles it interacts with. There's more to that of course, but you know what I'm getting at. To use "in pairs" in any other context is misleading because:
2) All other forces are net forces.
3) Net forces are composed of any number of those fundamental interactions of different kinds and in differing directions.
4) A force (actually a net force) between two objects is really a multitude of forces (of those individual fundamental forces). Calling that multitude a "pair" is nonsensical.
5) When we say "pair", what we really mean is the net force existing between the two objects concurrently pushes in one direction on one object and in the opposite direction on the other object. "Pair" applies to the two directions, but not to the (multitude of) forces adding into it.
Valuesize ( talk) 08:03, 28 September 2016 (UTC)
![]() | This ![]() It is of interest to the following WikiProjects: | ||||||||||
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Parts of the article have been flagged as needing citations. I've created this Talk Page Section so that editors can focus on that here.
Here is a complete re-write of this article. I have removed the stub pointer. Comments and suggestions are welcome. -- Michel M Verstraete 22:24, 21 June 2006 (UTC)
Dear Stephen, Thanks for your kind words, and apologies for the delayed response to your question: I only just saw your comment today.
The car crashing into a wall is subject to the same laws of physics as all other objects of the Universe, but this case is rather more complicated, because it involves not just objects moving with respect to each other but also deforming (to the point of breakdown). The simple answer is that the car will likely be destroyed too (or at least severely dented) in this experiment. If you replace the car by a cyclist, you will see that in that case the wall stands and the cyclist is hurt: it thus has to do with the strength or solidity of the objects that interact. The fact that the wall collapses does not imply it has no impact on the car.
A more detailed answer would go along the following lines: As soon as the car bumper touches the wall, it exerts a pressure force on the wall, and the wall exerts a similar (reaction) pressure force on the car. As long as these forces remain 'small enough', they only result in elastic deformations of both objects, following [Hooke's law] (Visualize a plastic bumper and a carton wall, for instance). As the pressure from the car increases, the reaction from the wall also increases. At some point, the structure of some of the materials involved will break down, the interaction becomes [Elasticity (physics)|inelastic], and the weaker one looses its ability to hold itself in one piece and thus also its ability to react. At that point, the car does not apply any force on the wall anymore (the contact point or surface has vanished) and thus the wall does not exert a force on the car either. The fact that the wall collapses is a subsequent consequence of the structural instability of the wall itself, following the piercing of a large hole and/or the shock wave induced into the wall; this has little or nothing to do with the car, as far as the action-reaction law is concerned.
I hope this helps. Michel M Verstraete 22:50, 5 June 2007 (UTC).
The article currently has a paragraph that reads:
I've never heard about this. Can we have a source for it? Hairouna 04:13, 12 September 2007 (UTC)
Think of it this way: While the definition of a force involves an accelerating mass, this thing does not normally exist in isolation. So if one billiard ball strikes another, at the moment of impact there is a force affecting BOTH balls. The simplest way to generically say it is, "A force typically comes into existence between two interacting things. It is the interaction that yields the force, and simply because both things are involved in a particular type of interaction, both things experience the same type of force."
All that said, I'm not sure I can agree with the claim that the preceding is always true for every possible kind of force. For example, an electron can absorb a photon, and acquire the kinetic energy and momentum of the photon. WHEN THIS EVENT happens, DURING it, the photon disappears and the electron experiences a force that accelerates it to a new velocity. I acknowledge this event properly belongs to the realm of Quantum Mechanics, where strange things are allowed (the electron may be described as literally instantly starting to move at the new velocity, and no Newtonian-type of acceleration/force may be involved at all). Nevertheless, this event does pose a problem with respect to the notion of Action and Reaction, simply because, while obviously the electron is Acting, after absorbing the photon, nothing exists that is Reacting! —Preceding unsigned comment added by 216.9.73.2 ( talk) 08:19, 7 January 2008 (UTC)
I've edited the explanation of one of the misunderstandings, to highlight where the confusion comes from. The force exerted by the table can be rightfully called reaction (and indeed that is often the case, as in
ground reaction), as long as it's clear what force it is a reaction to.
Also, I don't see why the table and the book are not at rest (in any inertial frame of reference). I think the statement implicitly assumes the Earth as reference frame, neglecting any effects due to its rotation, therefore both book and table are at rest, with respect to it. Anyway, the at-rest question is non-relevant to the 'weight-action/table-reaction' confusion and so I've removed it.
Giuliopp (
talk)
19:25, 17 November 2007 (UTC)
The statement "This is not the case, since the two forces are different in nature and are both applied to the book; . . ." cannot be correct. The book's weight is not applied to the book. A gravitational force is applied to the book. Weight is a force exerted by a massive object in a gravitational field (the book) on another object or surface (the table) that restrains motion toward the center of the gravitational field. No gravity, no weight. No restraint, no weight. Objects in free-fall have mass, but no weight. spottydog3 ( talk) 14:07, 30 September 2011 (UTC)
A new misunderstanding has been added. One way to test the description involves the toy Newton's cradle. It usually has about 5 suspended balls. Why do you never see a version of that toy with, say, 200 balls? (If you worry about accurate alignment, just replace all the center balls with a simple rod.) The answer relates to the time it takes the mechanical force to propagate to the last ball in the row. When the distance between first and last ball is short, the system allows essentially all the momentum to transfer easily from the first ball to the last ball. But when the distance is long, the first ball partly bounces back, and the last ball doesn't swing out as far as it does when the distance is short. The row of balls is acting like a massive solid object, NOT INSTANTLY RESPONDING AS A WHOLE TO THE APPLIED FORCE, until the last ball starts to move, and it is that "acting like a solid object" that causes the first ball to bounce. In this case, then, the action/reaction of the impact force between the first ball and the row occurs simultaneously, but the actual movement-action/reaction of the bodies is non-simultaneous, exactly as is described in the added "misunderstanding" text. —Preceding unsigned comment added by 216.9.73.101 ( talk) 23:30, 1 January 2008 (UTC)
Hi all,
I'm obviously not a math pro, the F=ma doesn't make sense to me. If there is an object at rest in space and has i.e. the mass of 10, then its force should be 0. F = 10*0
So if there are two objects placed near each other, both are at rest, what is the determining factor for their gravitational force. Mass only? Furthermore are the masses added up to form one force that acts on both objects? —Preceding unsigned comment added by 91.57.156.132 ( talk) 18:13, 12 March 2010 (UTC)
Since you have the clear idea about "A car driving in a curve exerts a centrifugal force on the road." I think you may just say, the "experiences" in the sample sentence "The centrifugal force that an object experiences is the reaction to the centripetal force on that object." could be replaced by "exerts to a rotation container". That will make it a clear statement. Jh17710 ( talk) 02:57, 9 January 2011 (UTC)
In the hammer throw example it states that "the athlete exerts an outward centrifugal force on the ball, the ball exerts an inward 'centripetal' force on the athlete". I'm not sure whether I don't understand the wording or this is wrong, but I'm pretty sure the athlete exerts the centripetal force on the hammer, preventing it from leaving the orbit in a straight trajectory. The ball pulls the athlete outwards and this pulling is counteracted by the inward pulling of the thrower. The person isn't in a position to exert a centrifugal force since centrifugal forces aren't exerted by an object, they occur in the a system due to the acceleration of the system.-- 129.247.247.238 ( talk) 08:00, 2 October 2012 (UTC)
I know that physicists can enjoy being pedantic, but there really are errors in this article, I'm sure, where someone has tried to make too big a deal out of nothing and ended up actually writing nonsense. Uncited and unreferenced nonsense. Here we introduce 'Examples of common misunderstandings' by saying "Newton's third law is frequently stated in a simplistic but incomplete or incorrect manner through statements such as [...] To every action there is an equal and opposite reaction". Yet this is the very wording that Newton's laws of motion uses, referenced to Newton's Principia, "To every action there is always an equal and opposite reaction", or in the reference, "To every action there is always opposed an equal reaction". [1]
Is the author here so clever that they are saying that Newton didn't really understand Newton's 3rd Law (but they do)? In the book on a table and the weight on a string or spring examples, a great deal of words are used to explain something terribly obscure and complicated, but later in the same reference, Newton wrote, "Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone". Here we expend a lot of hot air, but can we not just say that the book presses on the table and the table presses on the book?
If there is an understanding of Newton's 3rd Law to be had that is so subtle that it evaded even Newton, then I have to say that I have not found out what it is from all the verbosity, hot air and {citation needed} tags in this article. I enjoy proving generations of science teachers wrong as much as the next person, but I think we fall short of doing so here. Like the rest of Wikipedia, we should first find the references that explain why everyman is wrong, and then summarise the sources' arguments here - no more and no less. -- Nigelj ( talk) 14:55, 8 October 2011 (UTC)
This article focuses on misunderstandings of Newton's Third Law. As a college instructor in introductory physics, I appreciate the need to address such misunderstandings, but I doubt whether it should be done in an encyclopedia; certainly, it should not be the main substance of the main article. The main article should make a positive statement of the Third Law, together with a list of examples.
The various misunderstanding of the Third Law, with appropriate rebuttals, could be listed in a separate article.
Speaking of misunderstandings, the term 'reaction force' is used differently in physics literature. While introductory physics texts use it in the Third-Law sense, as reference to the other half of an interaction force pair, applied texts (e.g. biomechanics) mean by it the normal force by the ground on a supported weight. The current article correctly implies that the weight of the object and this normal force are not an action-reaction pair; this is easily seen from the example of an accelerated elevator, where the normal force has a different magnitude than the object's weight.
I am therefore not so sure that 'reaction force' is the best main title for this article. I would propose 'force pairs', or 'interaction pairs'. The fact that Newton himself spoke of actio/reactio does not impress me much. Surely, no one would suggest that we call inertia an 'intrinsic force' just because Newton did so in his 'Principia'...
In the article on Newton's Laws, some aspects of the 'reaction force' are addressed that deserve elaboration. For instance, Newton's Third Law is implied by the law of conservation of momentum. (If ∆p[system] = ΣF[external]t on one hand, but ∆p[system] = Σ∆p[parts] = ΣF[parts]t = ΣF[external]t + ΣF[internal]t on the other, then ΣF[internal] = 0 for any interaction.) Arjenvreugd ( talk) 01:56, 1 November 2011 (UTC)
The section currently says this:
F1. gravitational force by earth on object (downward) F2. gravitational force by object on earth (upward) F3. force by support on object (upward) F4. force by object on support (downward)
Forces F1 and F2 are equal because of Newton's Third Law; the same is true for forces F3 and F4. Forces F2 and F3 are only equal if the object is in equilibrium, and no other forces are applied. This has nothing to do with Newton's Third Law.
[end of quote]
In the penultimate sentence, should it refer to F1 and F3 being equal, rather than F2 and F3? (F2 and F3 are both acting upwards). 109.149.189.15 ( talk) 14:55, 17 October 2012 (UTC)
The statement that all forces come in pairs is a pretty common summary of Newton's third law. I don't think it's necessarily ambiguous, especially because the same sentence continues to qualify what constitute force pairs, ie if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. In the counter example provided by the editor removing the text, "a magnet pushes downward with its weight plus some magnetic component, but the earth pushes up with just one normal (electrostatic) force, that's 3 forces in balance. There's more to it of course, but "pairs" misleads", ignores the other forces in this interaction between the magnet and the earth. Going through the forces:
Note that none of the forces on the magnetic form a pair with any of the other forces on the magnet, which fulfills the second half of the "all forces come in pairs". Honestly, the sentence makes less sense to me without the "in pairs" comment - without it there is no motivation for the "such that" clause that follows. Can you explain why you see the pairs statement as ambiguous or misleading, because I'm not seeing it? What kind of can of worms does it open? -- FyzixFighter ( talk) 01:26, 28 September 2016 (UTC)
Hi FyzixFighter,
Your bulleted dissection showing that you understand supports my point perfectly. The bullets are excellent and correct. But, getting you and me to understand is not the goal here. Leading lay readers clearly (with reliable refs) is the goal.
The worst thing about "All forces come in pairs" is that it carelessly leads many lay readers to (incorrectly) conclude that there can never be a non-zero net force -- a problem I'm sure you're familiar with.
"All forces come in pairs" is frequently used by sources who don't really know what they're implying. Lot's of sources use the expression, and most of them don't use it clearly -- because they don't clarify what it doesn't mean. Many of them even mis-phrase it so it's just plain wrong.
It's way too easy to find an unreliable source that says "all forces come in pairs". Reliable sources aren't so careless as to go there. In fact, I'd go so far as to say that any source spouting a variant of "all forces come in pairs" should be considered dubious until a close careful look proves otherwise!
Your burden ( WP:burden) is to show that your sources are reliable. But even if they do explain it clearly without leaving open misinterpretations (which I truly hope they do), our text must also be clear in and of itself. Simply saying "all forces come in pairs" (even with reliable refs) doesn't achieve that. Luckily, there's an easy solution -- just leaving out "in pairs" altogether obviates the need to explain what it doesn't mean (with such long-winded explanations as the bulleted dissection and explaining that it doesn't mean forces always add to zero). "In pairs" just isn't necessary to get the point across, and there are are tons of reliable refs that don't say it.
Some thoughts for fun:
1) "forces exist in pairs" really only applies to the two directions of a force-carrying particle's transfer of momentum between the two particles it interacts with. There's more to that of course, but you know what I'm getting at. To use "in pairs" in any other context is misleading because:
2) All other forces are net forces.
3) Net forces are composed of any number of those fundamental interactions of different kinds and in differing directions.
4) A force (actually a net force) between two objects is really a multitude of forces (of those individual fundamental forces). Calling that multitude a "pair" is nonsensical.
5) When we say "pair", what we really mean is the net force existing between the two objects concurrently pushes in one direction on one object and in the opposite direction on the other object. "Pair" applies to the two directions, but not to the (multitude of) forces adding into it.
Valuesize ( talk) 08:03, 28 September 2016 (UTC)