Talk:Reaction (physics)

Citations Needed
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.
 * I'd like to try to find some references for one of the later flags. The last of the "Common Misunderstandings" describes how a force propagates through an object, in a way that is similar to sound waves.  Here is a reference regarding the propagation of sound waves: http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/elasticsolids.htm  Peculiarly, this page does not mention the original Force that had to be applied to an object, to initiate the propagation of sound waves through that object.  It seems to me that we editors are faced with a peculiar problem, with respect a particular thing that perhaps should be referenced.  Almost everyone has, at one time or another, placed an object next to an ear, and tapped on it, and heard the sounds of those taps.  Does anyone deny that those taps are Forces being applied?  Does anyone deny that if any one of those taps was applied when that object was isolated in Zero Gee, the whole object might start to move?  The knowledge of those two things is so common that I'm not sure where to find a Reference stating it!  This means it could be difficult to Reliable-Source support the statement that when a Force is applied to an object, that Force is responsible for both its motion and the sound waves that propagate through it.  Meanwhile, it is almost-as-widely-known that most ordinary events take some time to happen; Einstein would complain if anyone stated that the far end of the object must instantly begin to move when a Force is applied to its near end.  Forces do not propagate faster than the speed of light!  And only electromagnetic forces are able to propagate at the speed of light.  Mechanical forces, for example consider both the S-wave and the P-wave generated by an earthquake, travel far slower than the speed of light.  The speed of sound through the Earth is known to be the correct correct speed for those waves ( http://autocww.colorado.edu/~toldy2/E64ContentFiles/EarthSciences/seismology.htm ).  And, of course, it is also widely known that when an earthquake happens, the local ground doesn't move until the shock waves arrive --at the speed of sound in the ground!  I welcome any assistance in finding more generic References, for the propagation of motion-causing Forces in other materials! V (talk) 08:01, 7 August 2011 (UTC)
 * Here's something that was posted (below) a while back, which I had forgotten, and am now copying to here: Relevant to that last misunderstanding is this article: http://nextbigfuture.com/2009/12/update-on-rarefaction-wave-gun.html  Carefully note that the "rarefaction wave" discussed is actually a speed-of-sound-in-hot-pressurized-gas wave.  It is a reduced-pressure thing that, if it reached the projectile before the projectile exited the gun barrel, would lessen the force causing the projectile to be hurled.  In essence, then, the action of opening the breech while the gun is being fired does not instantly have the physical reaction of affecting the acceleration of the projectile, IF the opening takes place at the right moment (not too soon!) after the propellant charge is ignited.  The distance between the accelerating projectile and the breech, when the breech is opened, causes the same sort of delay as described above for 200 balls, in the effect of the simultaneously-appearing force that causes physical action and physical reaction. V (talk) 05:22, 26 August 2011 (UTC)
 * Aha! Perhaps this link is the most relevant so far: http://www.scribd.com/doc/52425003/Engineering-Fundamentals-of-the-Internal-Combustion-Engine-Willard-W-Pulkrabek  Somewhere in there should be an explanation of why internal combustion engines were modified to use an "overhead cam" instead of a "push rod" to achieve higher RPM.  But the simple explanation is that the Force applied to one end of a push rod takes time to propagate through the length of the rod (at the speed of sound in the steel of the rod), and that time delay is greatly reduced when an overhead cam instead directly affects the cylinder valves. V (talk) 16:29, 8 September 2011 (UTC)
 * In looking for more references along that line, there is this one: http://www.musclecarclub.com/library/tech/engine.shtml and this quote: "Some engines have the camshaft mounted above, or over, the cylinder head instead of inside the block (OHC "overhead camshaft" engines). This arrangement has the advantage of eliminating the added weight of the rocker arms and push rods; this weight can sometimes make the valves "float" when you are moving at high speeds." That Web Page appears to be written for casual readers instead of physics students; I'm pretty sure that the word "weight" in that quoted text should actually be "inertia". V (talk) 18:41, 11 September 2011 (UTC)
 * In this link http://gicl.cs.drexel.edu/wiki/Group_6_-_GM_4_Cylinder_Engine_-_1 there is a comparison of push rods vs overhead cams, and this statement: "An OHC design decreases play, or “float”, in the system" --that thing called "float" needs some attention. In this Patent: http://www.patents.com/us-6308677.html is this description: "While a cam in cylinder head of the engine eliminates the push rods that .... rely on springs to close a poppet valve, which, at high RPM, can lag behind the piston. ... causing valve float" --So, "float" is related to "lag", which is equivalent to a "time delay".  Well, the speed of sound and the length of a push rod are fixed, meaning that there is an inherent time delay built into a push-rod engine.  There is no way to eliminate that time delay, when you want the engine to run at very high R.P.M., without also eliminating the push rods. V (talk) 19:42, 11 September 2011 (UTC)

Rewrite
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)


 * This is an excellent article, I must particularly thank you for concentrating on the common misunderstandings of Newton's Third Law, as I suffer from them myself. I agree, I think that teachers and writers usually express it too curtly, in ways that allow incorrect ideas, and, worse, they don't take care to steer people away from those incorrect ideas. When people then go away and try to think about the Third Law they sooner or later trip up over contradictions and end up thinking they cannot understand the Third Law.
 * I wonder if you could add to your "Examples of common misunderstandings" a case that has long puzzled me: if a car drives through a brick wall, the car has applied a force to the wall, but the wall seems not to have applied an equal force back on the car, because the wall has collapsed. Stephen Fennell 217.33.113.3 10:09, 17 May 2007 (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).

Action and Reaction Must be of Same Physical Nature?
The article currently has a paragraph that reads:


 * Another important point to keep in mind is that the physical nature of the reaction force is identical to that of the action itself: if the action is due to gravity, the reaction is also due to gravity. Hence, any discussion of this topic that amounts to a claim that an action results in a reaction of a different type (gravitational, electromagnetic, friction, spring, or whatever) is obviously wrong and should be discarded.

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)

Book on the table
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)
 * I think the name of "contact force" in the sentence "In fact the force exerted by the table can be seen as the reaction to the contact force exerted by the book on the table, which in turn is equal to the book's weight." is not as good as simply the original name, "weight" of the book. The original name is not for the "gravity force" that the book applied to the earth, it is for the force that the book applied to a scale, any kind of scales, so that when we use the same name for the desk situation there is no misunderstanding.Jh17710 (talk) 02:38, 9 January 2011 (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)

Newton's cradle
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)
 * Relevant to that last misunderstanding is this article: http://nextbigfuture.com/2009/12/update-on-rarefaction-wave-gun.html Carefully note that the "rarefaction wave" discussed is actually a speed-of-sound-in-hot-pressurized-gas wave.  It is a reduced-pressure thing that, if it reached the projectile before the projectile exited the gun barrel, would lessen the force causing the projectile to be hurled.  In essence, then, the action of opening the breech while the gun is being fired does not instantly have the physical reaction of affecting the acceleration of the projectile, IF the opening takes place at the right moment (not too soon!) after the propellant charge is ignited.  The distance between the accelerating projectile and the breech, when the breech is opened, causes the same sort of delay as described above for 200 balls, in the effect of the simultaneously-appearing force that causes physical action and physical reaction. V (talk) 18:05, 18 December 2009 (UTC)

Can someone give a calculated example?
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)


 * First, please note that the equation F=ma is not about appearance of a force. It is about how an object responds to a force that is already happening.
 * Next, all known forces require some sort of interaction to take place for the force to exist, and an "interaction" is not a thing that only involves just one object. For gravity, you should probably look up the article on that subject (gravitation).  One relevant fact is that Newton independently invented calculus in order to show that the "center of mass" was a useful concept, when computing gravitational forces.  That is because the distance between two interacting objects (such as Earth and Sun) is also a factor, but if each object occupies a significant volume (the Sun is volumous enough to hold a million Earths), then what is the distance between them, to use in calculating the force?  For practical purposes we can use the distance between two actual points, each being the center-of-mass of an object.  The force that appears, when two massy objects interact, is described by Newton as F=Gm1m2/d2, where m1 and m2 are the two masses, d is the distance between them, and G is a special conversion factor ("gravitational constant") that allows the force to be described in standard units.
 * Finally, once the amount of force has been computed, you can now apply it in terms of the equation F=ma to see how much either mass accelerates as a consequence of the existence of that force. V (talk) 18:16, 12 April 2010 (UTC)

centrifugal force
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)

Pedantry and error
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".

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)
 * Nobody is suggesting that Isaac Newton misunderstood the Third Law or failed to appreciate a subtlety. The problem is that Newton's original language, when read by modern students, is often misunderstood. In part this is caused by the conciseness of Newton's statement: By "actio" and "reactio", Newton meant the forces exerted by two objects on each other. Another root of the problem is that this statement of the Third Law is often quoted without the explanation that follows in Newton's own text. In the Principia, Newton immediately gives a clear example and the warning that the forces, not the accelerations, are equal. And then there is 400 years of experience in teaching physics, in which it became clear that (pedagogically) better formulations of the principle are possible. That is not pedantic.
 * So the problem is not that Newton or others are wrong, but that non-specialists often misunderstand the principle formulated. You will find a discussion of these misconceptions, directly or indirectly, in any good physics textbook between 1700 and today. Arjenvreugd (talk) 21:26, 6 February 2012 (UTC)

Suggestions for clean-up/rewrite
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)


 * I implemented some of these changes. -- Arjenvreugd (talk) 23:17, 5 May 2012 (UTC)

Error in the 'supported mass' section?
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)


 * I agree. It's not a big mistake since F1=F2 and F3=F4 anyway, but where we're talking about whether the object is in equilibrium we should consider the forces on the object, which are F1 and F3. Fixed. &mdash;&thinsp; H HHIPPO  16:11, 17 October 2012 (UTC)


 * Thanks for the correction. Arjenvreugd (talk) 15:52, 22 October 2012 (UTC)

all forces come in pairs
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:
 * weight of the magnet is the gravitational force of the earth on the magnet
 * forms a pair with the gravitational force of the magnet on the earth
 * normal (electrostatic) force of the earth pushing up on the magnet
 * forms a pair with the force of the magnet pushing down on the earth (also electrostatic)
 * magnetic component of force on magnet (I'm assuming this is from the earth's magnetic field)
 * forms a pair with magnetic force on the earth from the magnet

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)

"Pairs" introduces confusion. Unreliable sources overuse it.
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)


 * Simplify and clarify, yes, but not to the point of being wrong. As I said before, I think the sentence in questions makes less sense without the "in pairs" comment. If all the sentence said is that all forces come in pairs and had no further clarification or description, then I would agree with you. However, I don't see how, when the sentence is read in its entirety, that a reader will make that mistake - the sentence as it has been for awhile qualifies the "in pairs" comment as being according to the 3rd law of motion and then explains what kind of forces qualify as a pair (different objects, equal magnitude, opposite direction). A reader has to completely ignore the rest of the sentence to reach the misinterpretations you describe above. I've provided a citation hence satisfying WP:BURDEN. I believe the source satisfies the requirements of WP:RS - that we disagree with a source is insufficient reason to declare a source "unreliable". While there are plenty of sources that don't use this description, that doesn't really say we can't, especially when there are reliable sources that do. I think a more important question is are there any sources that say that this description is misleading or pedagogically insufficient? --FyzixFighter (talk) 12:37, 28 September 2016 (UTC)