Physics&Parsimony

About physics and teaching

Confusion Regarding Newton’s Third Law of Motion

with 17 comments

Recently on the Modeling Instruction listserv there was a discussion about difficulties that students have with Newton’s Third Law and how teachers might best address these difficulties. It appeared to me (and I contributed to the discussion saying so) that some of the language being used by teachers was leading to  student confusion. It also appears to me that previous student experience with how the the Third Law is commonly explained (even well before high school) might be responsible. Finally, there are several different difficulties that students might be experiencing (arising from different conceptual issues) when they fail to use Newton’s Third Law correctly.

In this post I will address how Newton’s Third Law is commonly taught in high school (as well as introductory college courses), what the Third Law actually says, what it means and how all of this is often not at all clear to  students (or even teachers!). I’ll save for a separate blog post discussion of ideas on how to address student difficulties with the Third Law. In case you don’t have time to read all of this post, here is the short version:

  1. Newton’s Third Law is taught in first year courses as The Principle of Reciprocity, not as a law of motion. While this is a possible source of confusion, I believe it is still a good idea.
  2. The popular version of the Third Law involving the words “action” and “reaction” is actually a law of motion. The words action and reaction as used by Newton do not refer to forces. The use of the phrases “action force” and “reaction force” is a definite source of confusion.
  3. Much of the easily available information on Newton’s Third Law is presented incorrectly, and serves to reinforce student misconceptions or even confuse students further.
  4. Students have heard the Third Law presented as the Principle of Reciprocity disguised as a law of motion by teachers who don’t understand the difference, yet students think this is the one law they really do understand (since they can recite it) and are thus loathe to give it up!

How We Teach the Third Law

In first year courses, Newton’s Third Law is usually taught as a relationship between the forces two objects exert on one another. These forces are a result of a single interaction between the two objects (for instance, a gravitational interaction). One precise description of this particular manifestation of the Third Law is something like:

If object A exerts a force on object B, then object B exerts the same kind of force on object A, with the same magnitude and opposite direction.

As stated above, this Law says nothing about the motion (or changes in motion) of either object, yet it is the third of what are often called “Newton’s Laws of Motion.” Already we start to see why students get confused. Often students try to use the phrase “same magnitude and opposite direction” to attempt to come to some conclusion about the motion of a single object, leading them to falsely assume both forces in question are exerted upon that single object.

The concept stated above is more accurately described as “The Reciprocity Principle,” in that it describes a relationship between the two forces involved in a single interaction.  The Reciprocity Principle, as so stated, is not even universally true. Take, for instance, two protons, proton A moving in the positive x-direction, proton B moving in the positive y-direction. Calculate the instantaneous electric and magnetic forces that proton A exerts on proton B. Now calculate the instantaneous electric and magnetic forces that proton B exerts on proton A. Do this and you’ll see the point–there exists no reciprocity for those magnetic forces between individual charged particles!

So was Newton wrong? I’ll comment on that in the next section. For now let’s concentrate on why we would phrase Newton’s Third Law of Motion in a way that isn’t a law of motion, and doesn’t actually hold true for magnetic forces. The point of introducing Newton’s Third Law this way is that the Law of Reciprocity is a very good model for almost all interactions we encounter during a first year of physics at the high school or college level. The Principle of Reciprocity gives our students a tool with which to attack more complex situations involving several objects interacting within a system. As a bonus, we give ourselves a nice way to introduce the Principle of the Conservation of Momentum.These are good pedagogical reasons to teach the Principle of Reciprocity, rather than the Third Law as stated by Newton.

What Did Newton Actually Write?

Newton’s Third Law as found in Principia Mathematica is:

Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.

Great, it’s in Latin. Not to fear, Wikipedia has the following translation (Wikipedia article on Newton’s Laws of Motion) which is not attributed:

To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.

This is somewhat the version that is often quoted, leading to explanations involving “action forces” and “reaction forces.” After the confusing stuff about action and reaction (just what does Newton mean by these words? what do WE mean by those words?), it says something about forces. But wait. Just below this translation in the same article, a different, yet this time attributed, translation is given:

LAW III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

What are we to make of this? The second translation of the same text does not include the word “forces,” but rather uses the word “actions.” I consulted a Latin scholar far more erudite than myself: my son. He pointed out that the word Newton uses for force (vis) is not at all present in the above statement. Thus the second translation seems to more accurately reflect what Newton wrote. Still, what does Newton mean by “action?” Let’s look to the paragraph following this sentence (again, the translation is taken from the same Wikipedia article and the same attributed source):

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. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinges upon another, and by its force changes the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. For, as the motions are equally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions, as will be proved in the next scholium.

Gracious me. Newton’s explanation starts out talking about forces and pressures, speaks of obstructing and advancing “progress,” and finally ends up talking about what appears to be momentum. I’ll say right here that I am unimpressed with the clarity of Newton’s explanation. I go back and forth between thinking action/reaction should be read as the change in motion as opposed to being read as the thing that changes the motion. But rather than pick nits (and criticize an author who can’t defend himself), let’s zero in on what Newton seems most intent upon telling us in this, his third law of motion: there is a specific relationship between the changes in the motions of two interacting objects. Hence, this is a law of motion.

In fact, it appears that Newton’s Third Law is a statement of the conservation of momentum for the situation of two isolated (“if the bodies are not hindered by any other impediments”), interacting objects. Although Newton didn’t know about field momentum, the momentum interpretation of the Third Law almost works for the case of two interacting protons that I cited above. If you take proton A, proton B and the electromagnetic field as your system, the momentum of the system is indeed constant, but this is no longer a two-body problem. The Principle of Reciprocity, by focusing on forces rather than momentum, misses the momentum that is gained by the electromagnetic field.

Besides noting that Newton’s version of the Third Law being an actual law of motion, I’d like to make the case that the terms “action” and “reaction” should be dropped because of the confusion they cause. Regardless of what Newton meant, today these words have no meaning in terms of forces or changes in momentum. Some of the discussion on the Modeling Instruction listserv revolved around students mistaking the “reaction” for the change in motion. Well this makes perfect sense, given our current understanding of the word reaction! We could clearly define the words action and reaction for use in the physics community (much as we clearly define work, energy, momentum, etc.), but we rarely use these words except to talk about the Third Law. Introducing new terminology only to abandon it soon afterwards is simply confusing to the student. Don’t do it. The only good use of action/reaction is for the name of a blog.

What Can Students Find on the Internet About the Third Law?

Sadly, much of the help for Newton’s Third Law that is available for students on the internet is poorly explained or just plain wrong. The confused student is very likely to seek help where help is supposed to be. Why wouldn’t a student struggling with Newton’s Third Law not simply pop “newton’s laws of motion” into a search bar? If they do, they will find the world stacked against them.

The first hit on just such a Google search results in a college site (must be vetted, no?) that gives the standard unhelpful  “For every action there is an equal and opposite reaction” followed by a single example that talks about motion (not forces, not changes in motion, but motion). This example does nothing to help explain the Third Law. So the student keeps looking. “Hey, next is rice.edu? Mr. Hammond went to Rice, this should be good.” The same unhelpful phrase is given followed by an example of a rocket. However the example misidentifies the appropriate force-pair and seems to indicate a rocket cannot accelerate unless it has the ground to push off of… confusing. Go on to the third hit, which leads to Wikipedia. Lots of symbols, calculus right off the bat, Latin, blah, blah, blah, nothing helpful to the beginning student. The fourth hit: NASA. Now we’re making progress! But the worksheets from NASA all include the misconception that both forces described by the Third Law are exerted on the same object! That’s right, the NASA site is dead wrong! Fifth site: Discovery Channel! Yay, TV! Yet the physics is all wrong, making the same mistake as the NASA site. Sixth site: physics4kids.com, a terrible website full of incorrect physics, including an incorrect explanation of the Third Law (same mistake as the NASA site made). Finally, on the seventh and eighth hits, we get some explanations that are correct and might be useful to the beginning student. How many students are going to get past the unhelpful and incorrect explanations (which probably coincide with their current misconceptions)?

Oh, you can go look at Khan Academy. At least they changed their wrong explanation after some physics teacher complained (who was that?). But for reasons I’ll explain in my follow up post to this one, passive explanations don’t really get you too far unless you are already almost there.

What Do Kids Bring to the Classroom Regarding the Third Law?

Unfortunately, the frequency with which I hear students spout “for every action there is an equal and opposite reaction!” whenever they see any two forces pointing in opposite directions with equal magnitudes tells me that this kindergarten version of the Third Law is omnipresent. The students are presented with this version in middle school or on TV science programs, and they have probably had at least one confused adult give an incorrect explanation of what it means. For such a widely memorized tidbit, there sure is a lot of misunderstanding about what it means! To make things worse, many kids come into physics thinking that this is the ONLY bit of physics they already understand!

In many school settings, the Third Law is stated simply as “for every action there is an equal and opposite reaction.” This might be fine if the explanation that followed developed action and reaction as changes in momentum. But nearly universally, this version of Newton’s Third Law is followed by a lot of talk about “action forces” and “reaction forces” and very little talk about what objects are exerting forces and what objects are being subjected to forces. Most middle school textbook explanations I’ve seen are simply impossible to understand.

Thus students tend to make the following mistakes regarding the Third Law:

  1. They believe that the force a larger object exerts on a smaller object is larger than the force the smaller object exerts on the larger object. A big truck must exert greater force than a small car. A long rope must exert a greater force than a small rope. (Force-pair relationships are effected by size.)
  2. They believe that the force exerted by a fast object, when colliding with a slow (or stationary) object, must exert a greater force than the slower  object exerts on the faster object. (Force-pair relationships are effected by relative speed.)
  3. They buy into the Third Law only for objects at constant speed. (Force-pair relationships are effected by absolute speed.)
  4. They think that the law refers to forces exerted upon (and the motion of) a single object. This mistake is understandable given the massive amounts of misinformation to which the students have been subjected. (Force-pair misidentified/not identified.)
  5. They think that the Third Law no longer applies when one of the interacting objects breaks. (Force-pair relationships are effected by the strength of the materials involved.)
  6. They can properly use the Third Law, but don’t believe that it works in the “real world.” (Physics world and real world do not overlap. Getting an A means saying some stuff just for the teacher’s benefit: “Those forces are equal and, yes, Beloved is my favorite novel.”)
  7. They can state and use the Third Law, but fail to see when it might be helpful in various contexts. (Poor transfer of knowledge.)

I have some ideas about how to address student misunderstandings of the Third Law which I’ll share in a follow-up post. Some of the students’ misconceptions are actually astute observations hindered by lack of a consistent conceptual framework for incorporating those observations. That is, rather than just being ignorant, the students are trying quite hard (and almost successfully!) to make sense of the world. This should be a very strong position from which to start!

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Written by Mark Hammond

2011/12/22 at 20:43

Posted in Uncategorized

17 Responses

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  1. I have to throw my momentum-is-king hat into the ring here. I think Newton’s 3rd law is the most fundamental thing in the universe: Things in an interaction swap momentum. The 2nd law simply gives us a name for the rate of the swapping. The only other thing we need to know about the universe is what sort of interactions are possible (so you can get the swap rate as a function of distance). I’ve had some success working with pre-physics-service teachers with this approach and it is mostly grounded in the “6 ideas” curriculum.

    For me, N3L didn’t mean much until I started thinking about it this way. I really like how you try to focus the conversation about it as a law of motion, as opposed to reciprocity, Mark. My approach does the same thing, I think.

    By the way, just to finish up the laws: “If you’re not in an interaction, you don’t swap momentum” is my version of N1L.

    Andy "SuperFly" Rundquist

    2011/12/22 at 21:30

    • I’ve toyed with the idea of starting with momentum in my first year class… but haven’t gone there yet because I feel the obvious and concrete nature of velocity is a good place to start with 15-16 year-olds. Now my second year class is different. We start with momentum and make it king. I think we could talk more in terms of transfer of momentum in that course though… I’ve noticed a bit of a disconnect between what we do in the computational side of the course (updating momentum assuming constant force over a short delta t) and how they think of the momentum principle for constant force situations (where you don’t need computational models). Interesting.

      In our first year class, Kelly and I started talking about momentum transfer instead of just momentum conservation. We could never agree… does one say the momentum of a system “is not conserved” if there is an net external force, or do you say momentum is always conserved, as the system’s momentum change is offset by the surrounding’s momentum change? I vote for “momentum is always conserved, but it can change in a system.” Confusing? Or is saying momentum is “not conserved” in some net force situations confusing? This makes it sound like conservation of momentum is a special case, not a fundamental principle. Anyway, saying momentum is transferred (and conserved!) is the answer!

      Mark Hammond

      2011/12/23 at 11:17

  2. “Some of the students’ misconceptions are actually astute observations hindered by lack of a consistent conceptual framework for incorporating those observations. That is, rather than just being ignorant, the students are trying quite hard (and almost successfully!) to make sense of the world. This should be a very strong position from which to start!”

    I agree that they are trying, hard even, but the way they are trying is deeply flawed (but natural), at least as it relates to what is going on in physics. Your strong starting position is essentially that you are starting from scratch.:) And I wouldn’t call their observations “astute”, yet. To me, astuteness implies observing in a way that is conducive to notions like force, mass and inertia. In fact, astuteness is something most of them will have to develop, with your help, if they are going to be successful. And when that fails I don’t think it is simply a matter of poor language or poor examples (assuming that the whole curriculum isn’t garbage). As you said…

    “But for reasons I’ll explain in my follow up post to this one, passive explanations don’t really get you too far unless you are already almost there.”

    No choice of words will get you anywhere unless you are already almost there. In fact, current work in natural language processing is proving that out. How well a dialog works depends more on the listener than on the speaker. In order for these dialogs (in physics, or anything for that matter) to work, the student must be “almost there” at each step, and you can’t circumvent that requirement no matter how perfect your language. They must be following. They cannot be passive in mind.

    When that fails it fails because the student is unfamiliar with their side of the bargain in these dialogs. They don’t think like that. They are not, nor have they ever really been, analytical. I think what is lacking is a natural tendency to analyze and figure things out. Some people study things, just out of habit. They are naturally inclined to astute observation and analysis. Some students though are unfamiliar with those inclinations and subsequently have very little (if any) experience with the type of analytical thinking required for their side of the conversation. You are literally starting from the position of having to teach them to think period, let alone about anything in particular like physics.

    I think your ideas here are great, for teachers who are inclined to thinking about the clearest ways in which to present things and language is certainly a big part of that picture. But when a student describes force incorrectly I don’t think it is because they were given the wrong definition of force, I think it is that they simply don’t understand force itself. And I would think that no matter how nice a definition they recite IF I thought they were simply reciting it (by rote). What I am trying to say is that when language appears to be the problem, then that it is probably exactly what is not the problem, because that implies rote recital which means that the actual problem is lack of understanding.

    In that light I caution against too much emphasis on “perfect” definitions of these things because language is actually meant to talk to these things, not precisely define them. Too much emphasis on the latter can indeed instill rote recital. I know that some here will say “but what are definitions then?” and I will reply, they are certainly not the end all because if they were then all we would need is a dictionary and we would know everything. Definitions are actually an invention, a tool in fact, to help us with words and vocabulary. They aid us in being articulate but they are not meant to teach theory and understanding. A student can understand force but not be very articulate in expressing that understanding. That is where definitions and vocabulary come in. They are there to make the student articulate, not knowledgable. You need the understanding absolutely and you need the language to communicate that understanding, but they are not the same thing.

    Robert Hansen

    2011/12/23 at 06:44

    • I think we are using the word “astute” differently. What I consider an astute observation coming from a 15 year-old just starting to study physics I would consider banal or even ignorant coming from one of my second year students. Perhaps I should be less emphatic and say that some of my beginners’ observations are good and potentially useful, but not yet part of a coherent explanation of the world. Case in point: Beginning students often see a block sliding across the floor, slowing down, and say that the force I gave the block is slowly being drained by the floor. This is wrong, of course, but what is “astute” here is the observation that something is being “drained” from the block. It takes some time for them to build up the conceptual framework and the deep understanding of what we mean by “force” to see that it is momentum that is being “drained” (or transferred) out of the system.

      I agree with you that seeking perfect definitions is a useless exercise. You can’t just explain physics to students, otherwise they just “learn about physics” (maybe, if you are lucky) rather than “learn physics.” The point of my post was not to perfect an explanation of Newton’s Third Law. My point was to address confusion (as much amongst teachers as students!) about what the law says and how we teach it. Every kid I teach can recite the law after one class; however, almost none can use it. After working with the concept for some period of time (a different period of time for each student) do they start to be able to use the concept in a useful way within their slowly growing overall conception of the physical world.

      Yet I believe that their previous experience with the Third Law (and our popular culture’s confusion about the law) prepare the students in ways that make breaking through to understanding particularly difficult. Some of that confusion (amongst teachers as well as students) is due to the use of words that we do not agree upon, so the simple thing to do is to eliminate these (otherwise unused) words. Then it is important for the teacher (coach, guide, what have you) be aware of the subtleties involved (are you teaching a law of motion or the a law about forces?). A confused and garbled presentation is to be avoided at all costs, even though the explanation alone is not enough.

      In the end, though, it is the students’ explanations that really matter. I never really know what my students understand deeply until they talk at length about physics.

      Mark Hammond

      2011/12/23 at 11:38

      • “Beginning students often see a block sliding across the floor, slowing down, and say that the force I gave the block is slowly being drained by the floor.”

        I think we are on the same page, I consider that to be astute. The student is trying to factor what they observe into a theory. If a student simply says “the block is slowing down” then they are not being astute. I would agree that your version of “astute” is a great start. The only hurdle left is for the student to hold in limbo the foundational stuff (force, mass, weight, momentum, friction etc.) long enough for it to all come together.

        “In the end, though, it is the students’ explanations that really matter. I never really know what my students understand deeply until they talk at length about physics.”

        Agreed

        Robert Hansen

        2011/12/23 at 14:00

  3. One approach can be to point out that the dimensional units of Force x Time are those of momentum, and
    that applying a force F1 on an object for time T1, changes its momentum by P1 = F1 x T1. (The 2nd Law)

    In an isolated two-body (classical mechanical) system if body B1 exerts a force F1 on body B2, then in order to obey momentum conservation at every interval of time, no matter how small, body B1 has to have a force -F1 exerted on it, so that the P1 gained by body B2 is cancelled out by a -P1 change in momentum by body B1. Since the system is isolated, that force -F1 has to come from body B2.

    Admittedly this makes the 3rd Law sound like a consequence of the 2nd Law plus momentum conservation, but that is the case. Just as the 1st Law is just a trivial special case of the 2nd Law when acceleration is equal to zero.

    The Hindude

    2012/01/14 at 12:01

    • This is, in fact, similar to the way we approach things. Rather than taking an approach using dimensional analysis (noting that units work out the same), as soon as students have discovered for themselves that this strange quantity m*v seems to remain the same, if we put all the m*v’s together (in our system), I treat them to a system where m*v does NOT stay the same due to an outside influence. That influence happens to be a rubber bumper connected to a force sensor into which I run a low friction cart. We look at the velocity vs. time graph for the cart and the force vs. time graph for the rubber bumper/force sensor. I ask them how we might use the force vs. time graph (it’s not a simple shape) and most immediately say we should look at the area, since they can’t think of much else to do with such a non-constant, non-linear graph. We have the computer calculate the area under the curve and see that it’s units and magnitude match up very well with the change in m*v of the cart. Only then do I take the students back into the classroom and have them play with Newton’s 2nd Law until they find out that, indeed, F*deltaT should be equal to deltaP.

      Then I can make the argument (outlined in your second paragraph) that if we put two objects in the system (with no outside interactions), use the 2nd Law, cite the 3rd Law along the way, we get exactly what the students discovered by running carts into each other on a ramp. The students get a huge kick out of this. There is much rejoicing.

      As for the 3rd Law sounding like the 2nd Law plus momentum conservation, I would argue (and have in my latest post) that Newton was actually getting at momentum conservation in his statement of the 3rd Law.

      Mark Hammond

      2012/01/22 at 09:30

      • Having that force-sensor really changes the way you can teach this! So if you take the vel vs time graph and re-draw it as its own time derivative, it should look just like the force vs time graph in shape. At least proportional anyway.

        Regarding your comment in your final para, I suppose that’s true. So why not dispense with teaching the 3 laws and start with “there is this thing called momentum that is conserved no matter what else happens. Don’t worry about the “why”, you’ll get a sort of explanation for that if you keep studying physics for another 3 years! Just assume it is conserved and let us see what sort of results a little basic algebra can show us.”

        The Hindude

        2012/01/25 at 19:30

  4. Bringing the point about the flaw in explaining the 3rd law is great, I was over excited to know it clearly.But I don’t see any article as promised giving clear explanation, this is more cruel. At the end “look at my next article” the next one never came…….

    Krishna

    2012/06/17 at 23:25

    • For this, I apologize. I was very busy this spring, but I plan to get back to blogging soon! I promise I’ll start with how I have taught Newton’s Third Law.

      Mark Hammond

      2012/07/16 at 13:21

  5. I wan’t to prove newton third law WRONG that says:for every action there is equal and opposite reaction.actually this law is incomplete because it says for ‘every’ action there is equal and opposite reaction.”what about during the action/collusion between two objects that have different forces/mass that they apply to each other,will the reaction be the same?”Obvious that is the big ‘NO’ because an object that has less mass/force will also apply less force to an object that have great force,that means that “for every action there is unequal and opposite reaction.According to me,Newton’s third law should be state like this:”For every action they will be equal and opposite reaction,when the objects that are in reaction have same mass/force that is applied to each other.”

    Maokeng Thabang

    2012/08/06 at 10:41

    • Your disagreement with Newton’s Third Law falls under misconception number one in my list of typical student misconceptions in the last section of my article. It is not “obvious” that objects with different masses exert different forces on each other. In fact, it is experimentally verifiable that they DO exert the SAME magnitude force on each other. Experimental evidence will trump supposition every time.

      Mark Hammond

      2012/08/08 at 14:16

  6. I enjoyed your article very much. My view is that the third law is about changes in motion, not forces. There are always action/reaction pairs (I don’t like the terms either) or change in momentum pairs for the reasons you have mentioned and a few others. This is not necessarily always true for forces as you point out.

    This becomes apparent when dealing with an atom emitting a photon and that photon being (later) absorbed by another identical atom. The exchange of a photon causes the emitting atom to experience a force and change in momentum and the absorbing atom to experience an equal and opposite force and change in momentum but at a slightly later time. So force reciprocity is not present at all times, but conservation of momentum always is (since the photon carries momentum).

    I am currently engaged in a very interesting discussion with some very good physics minds on Physicsforums.com about the third law in the context of centripetal force and the proper characterisation of the “reaction” to a centripetal force. Since in a rotating system, there is only centripetal acceleration, my view is that the third law pair to a centripetal change in motion/momentum of one body in the system is an equal but opposite centripetal motion/momentum of the rest of the system. The other view is that the “reaction” pair to the centripetal force on a body in the system is a “centrifugal reaction force” of that body on the system. The suggestion being that this is a real force not to be confused with the “fictitious centrifugal force” but does not result in any “change in motion” at all. I am opposed to using the notion of a real centrifugal force as there is no real centrifugal acceleration. There is even a Wikipedia page on this “reactive centrifugal force” at http://en.wikipedia.org/wiki/Reactive_centrifugal_force , which I think is completely misconceived and mostly wrong. But I seem to be a voice in the wilderness on physicsforums on this issue.

    I would encourage you to keep up this interesting blog.
    Thanks again.

    Andrew Mason

    2013/02/02 at 01:08

  7. I am studying to be a new teacher and struggling with coming up with a “logic flow” for Newton’s Laws. Your post was very helpful, but I see you still have not posted the next article you promised Krishna. I would be very interested in hearing your ideas on how to scaffold students to a deep understanding of Newton’s Third Law.

    Rose Davila-Gay

    2013/02/26 at 12:42

  8. Thank you, I found this because I “knew” the law was “wrong” but what’s wrong is actually the vocabulary.

    Of course if I press my hand against a wall, my hand experiences the same force Im pushing towards the wall, but applied to my hand, which alters the shape of my hand. And if I bounce two balls against each other, each experience a force on themselves an change directions. But it’s about which body receives which force – and it’s a description of what can be viewed at plain sight, measured, etc…

    The whole action vs reaction and being equal… really poor wording. I’ve seen plenty of science people just repeat it as a mantra, because it’s the “law”. Thank you for taking the time to really get into the matter of it.

    YOHAMI

    2013/08/28 at 07:04

  9. I was quite impressed by this article but I would greatly appreciate it if you could clear the misconceptions which you have mentioned above and correctly explain the third law of motion. Its been more than a year since you wrote this article and there hasn’t been an explanation of the law as of yet…

    Anonymous

    2014/01/12 at 06:36

    • Thanks for the comment. I regret that I haven’t kept up my blog, but I would like to address the explanation of Newton’s Third Law. That explanation is in the first section of the existing blog post. That is, the very first quote contains a simple and proper statement of the Law. This quote is followed by a rather extensive explanation of what the Law is and isn’t. What I promised to write (and utterly failed to supply… sorry) is a description of how to lead students to a proper understanding of the Third Law. I do plan to write that; however, time is tight and blogging activities have sadly taken a backseat.

      Mark Hammond

      2014/01/12 at 09:04


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