Comment on Khan’s Newton’s 3rd Law Video



Link to video



  1. Kelly O'Shea

    Ah! Yes! Yes! Yes!

    This student is so ready to really get N3L. I’m really impressed that they’re bothered by that contradiction.

    I’d move over to the balky donkey problem and get them to start drawing FBDs. Hopefully for the donkey and for the cart and maybe also for the donkey+cart. Usually, by working through their thoughts to get to the correct FBDs, they start to get the idea that you need to look at all of the forces on one object to talk about the change in motion.

    • bwfrank

      The donkey question is one I go to often with students. For some students it is very productive; for others it is not. I’m trying to better discern those students who a ready to tackle the task from those students who are not… any ideas? Like what does a learner already have to have in place for the pursuit of that paradox to be generative

    • secretseasons

      I wonder why we immediately go to these horse/donkey+cart style problems, which involve, really, three objects (the friction and third-law forces from the ground are important). This student is stuck enough on just two objects. Why not simplify the donkey+cart problem down to a more basic one: How is it possible for me to walk across the floor? (Don’t bother making me drag a cart behind me.)

      • Kelly O'Shea

        I think the reason not to go to “How is it possible for me to walk across the floor?” is that it is not good to think of the person as a particle in that situation (and the student likely has very few, if any, resources for thinking about an object as not a particle). If they’re getting tripped up on the difference between looking at one force on an object vs all of the forces on an object, having that object have parts that move separately from each other doesn’t seem like a helpful thing. Walking might be the next thing to grab after the balky donkey, but I would bet on it (human walking) making the situation quite a bit worse before it got better.

        • Max Goldstein

          What’s happening is that the student is dogmatically applying the English version of N3L without a true understanding of it. In this case, the student is missing that the law always refers to TWO objects, with one force acting on each. That might be the sort of thing you just have to point out – when you do FBDs of one particle, you’ll only ever see one of the pair. Or ask them to do FBD of the pen and then ask why they’re putting the second force in.

        • Boris Ovetsky

          If you do not like walking, try riding a monocycle (one point of interaction with a floor) or jumping on one leg. Same idea, but less complexity.

  2. Boris Ovetsky

    I am shocked after watching this video. Khan talks about equilibrium of a foot on a sand and adds two forces that applied to different objects! I would give an F to a student in my physics class for such an elementary error.

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