My teaching nemesis: Newton’s third

I’m sure everyone has a topic they teach where they teach it, the students look blank, you teach it differently, still blank, you then do it even differently, same response. Doubt, panic, fear! This happens to me every time I’ve taught Newton’s third law and then tried to bring back in balanced and unbalanced forces on one object.

In isolation, the two topics are fine. It’s at the point where they need to hold both concepts in their minds together that they realise they just don’t get it.

I don’t get it, miss

I can’t do physics

This is too hard

I don’t understand

The worst phrases for me to hear in my classroom. I must have taught these two topics somewhere between 5 and 10 times, to different groups. I know I’ll be teaching it at least to two different groups next year.  It’s my teaching Achilles’ heel.

So why am I writing this and even admitting it? Well, firstly I’d like to reassure my employer that I get there in the end with my students. After a few lessons of working through it, exam questions, thinking, reflecting and (most importantly) giving them time to digest the new ideas and sort it out in their own minds, they understand the two topics. But I’d dearly like to be able to teach these topics in a way that doesn’t seem to require my students going through a confusing, doubt-their-own-ability phase. It doesn’t happen to anywhere near this degree with any other topic. I want to know if it’s just me? Is there a better way? is it just very difficult for year 10 to understand?

I’d also like to reassure other teachers that this happens (I hope?!) and we all (probably) have topics we find tricky to teach. Well, I do anyway.

So what’s the problem?

The two topics are very similar, but distinct. My students seem to have trouble distinguishing them. I’ve simplified it down to:

Interaction pairs: 2 objects, equal and opposite forces, forces are the same type and act on different objects.

Balanced forces: 1 object, forces can be different types, If they are not equal and opposite then you get acceleration.

The difficulty seems to be that the phrase ‘equal and opposite’ is thrown all over the place. I emphasise the fact that one situation involves looking at all the forces on 1 object, the other is about 2 objects interacting. We look at examples, do practical work. I’ve done it in different orders.

How can something be moving if the forces on it are equal and opposite?

Because the forces on that object are not all balanced. There is a resultant force on that particular object causing it to accelerate. The interaction pairs to the forces on that object are on other things. Look at the gravitational force, for example…

I can see why it’s quite confusing.

Is it just too difficult for year 10? I don’t think so. I see it’s challenging and makes them think. Perhaps it’s one of the first times they’ve had to actually understand a concept to be able to apply the rules to a particular problem? They try to apply the simple rules but find they need to figure out which of the two situations they are looking at and this is where a lot of them struggle. Last year I told my (very bright, top set) class that they would likely struggle with the next topic, but it’s ok, normal, and they will get there. They didn’t believe me, then everything I’ve described above happened.

Some students do get it straight away. Thank goodness.

I’ve talked to colleagues. They agree it’s tricky.

I’ve read lots of physics books in order to refine my explanations. I’m aiming for perfect clarity. I know I understand the concepts myself and this isn’t the source of the problem. (honest!)

I’ve read Making Sense of Secondary Science – Rosalind Driver et al and in here these issues are highlighted. I’ve followed the advice.

I’ve asked twitter for advice and shown my pupils the marvellous Veritasium video: Best film on Newton’s Third Law. Ever.  (i.e. when despairing get someone else to explain it 😉 )

Is there a better way?  Well, there must be. What is it? help me find a better way 🙂

NOTE: If you are unaware of the topics I’m referring to, you can find out more about them here using this link: forces. It is the BBC bitesize summary of the topic and you need to look at three pages.

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7 Responses to My teaching nemesis: Newton’s third

  1. Neil Williams says:

    Apologies if this isn’t the case for your students. One problem mine were having was the quality of their force diagrams. They were so used to using ‘floating’ forces from KS3 that they couldn’t appreciate the importance of the precise location of the forces involved. They were assuming that all forces drawn impacted on the subsequent motion of ‘the’ object rather than just those acting on ‘the’ object. Hence they were struggling with how there could ever be an imbalance to cause acceleration. Modelling an example of a car accelerating to a constant speed drawing forces at each stage helped with this, a little! (We’ve also adapted KS3 schemes to focus more explicitly on the accuracy of force diagrams). On a slightly different note, it’s helpful for students to struggle and get through it and no matter how clear the explanation they’re going to have to go home and think about it – especially if they go on to study at A-level or beyond! Anyway, hope this has been some help…


  2. audsley says:

    Hi, Thanks for your comment. I think the quality of force diagrams is something that could be improved and it could be done better at KS3. I’ll look at working this into KS3 schemes (all being rewritten this summer so it’s a good time!) and I can dosome work with next years 10s at the start of this topic. Thanks for the suggestion.


  3. Helen says:

    Agreed, I find this really difficult to teach until it is sorted out with a lot of exam question practice and then they get the patterns involved. But my descriptions, I think because they are difficult to visualise, do confuse some less confident students at first. Good unpicking of why. I struggle partly because it’s so intrinsic to my own understanding that I don’t get why they don’t get it, and assume too much sometimes.


  4. audsley says:

    Thanks for the comment. its good to know it’s not just me and my students. It is a challenge to find a different angle to explain it from when the explanation that makes the most sense personally has not worked. I keep reading different books to get different ways of explaining it.


  5. It’s great of you to be willing to own up and say you have a problem area, but I do wonder whether you are beating yourself up unnecessarily. When you asked on Twitter, my rather flippant reply was “Don’t teach it”, but actually having read your piece, and taking into account your twitter reply to the effect that the spec. requires it I still think “don’t teach it”.

    My kids all come to me knowing Newton Three – they can quote it – even if they don’t know that it is Newton’s Third Law of Motion. And I’ve yet to meet a child that doesn’t understand that if you push against something it pushes back – so force pairs (the most that is likely to turn up in an exam) are no problem.

    The problem comes if you try to teach it with the other two Laws of Motion, because as you’ve said you’re teaching that forces look like they cancel out at the same time as teaching that a resultant force is needed for a change in motion.

    You can do it:

    “if I accelerate a block by pushing it, the block is still pushing back on my hand, in fact the forces between the block and my hand have to be equal and opposite because there is no acceleration between the two.”

    But that relies on them understanding relative motion, reference frames and having accurate representations of forces in their mental models, and they still want to know where the resultant has come from!

    If something is too tough to explain well and have them understand – cheat – sidestep it – give them the bare facts, not elaborate and misleading explanations (or even worse some half baked analogy), and rely on them understanding it later if they take Physics further.

    At least that’s what I do. If I teach NIII at all, it is with statics, when we discuss the nature of forces – “Why doesn’t the book fall through the table?”. When I get to the laws of motion I deliberately de-emphasise it. I describe it as the “most remembered and most useless law they’ll meet in school Physics” and leave it at that.

    Liked by 1 person

    • audsley says:

      Thanks for that.It really makes a lot of sense. I’ve never honestly considered just not teaching it as you’ve described, but you make a great case for it! Especially, as others have said, it isn’t examined particularly, if at all.


  6. dodiscimus says:

    I think it’s the most conceptually challenging bit of classical physics (and I don’t just mean in GCSE). The response from @SteveTeachPhysics is interesting. I too think there is a valid case for just paying it lip-service at Y10/11. If you look at the new AQA spec it’s noticeable that there is just the one bullet point on the 3rd Law and much more on the 1st and 2nd. Sometimes I think it’s okay to plant a little seed to lie dormant, if the GCSE allows it. I would want my students to know the law, and to be able to handle the idea that a tyre pushes backwards on the road, so the road pushes forwards on the tyre (as an explanation for where the forward force on a vehicle comes from), and that if you have something like a ladder leaning on a wall then the wall pushes back on the ladder, but his last example of a book on a table is, for me, a place I wouldn’t want to go until I was sure the students were ready. If you do this then you have to deal with the idea that the contact force of the book on the table is NOT the weight of the book although it is the same size and direction as the weight of the book. Then you have to bring in the gravitational force of the book on the Earth and the bewildering fact that the pull of the book on the Earth is the same size as the pull of the Earth on the book (and that you can’t really call them both weights, which adds a naming problem on top of everything else). With a lot of thought most physics teachers might be able to manage a clear explanation that doesn’t muddle the two separate pairs of 3rd Law forces, and still has the book in equilibrium, but I don’t think typical students have a gnat’s chuff of a chance of understanding. And don’t forget this is all on top of Newton’s 1st Law, which by itself is a minefield of conceptual challenge.
    Best wishes


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