Talk:Reaction (physics)

message: 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)
replies: text: 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)
replies: text: 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)
replies: text: 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)

text: 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)
thread_title: Citations Needed

message: 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)
thread_title:  Action and Reaction Must be of Same Physical Nature? 

message: 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)
replies: text: 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)
thread_title:  Book on the table 

message: 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)
replies: text: 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)
thread_title: Newton's cradle

message: 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)
thread_title: centrifugal force 

message: 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)
replies: text: I implemented some of these changes. -- Arjenvreugd (talk) 23:17, 5 May 2012 (UTC)
thread_title:  Suggestions for clean-up/rewrite 

message: 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)
replies: text: 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. — HHHIPPO 16:11, 17 October 2012 (UTC)

text: Thanks for the correction. Arjenvreugd (talk) 15:52, 22 October 2012 (UTC)
thread_title:  Error in the 'supported mass' section? 

message: 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)
replies: text: 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)
thread_title:  all forces come in pairs 

message: 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)
replies: text: 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)
thread_title: "Pairs" introduces confusion.  Unreliable sources overuse it.