Use less force and more interaction to understand Newton’s 3rd law
I’ve written before about how Newton’s 3rd law is a difficult concept to understand, and the ideas of “action/reaction” or “equal and opposite” don’t really do much to aid understanding. Previously, I’ve taught students this mantra to replace the whole “action/reaction” misconception:
If object A exerts a force on object B, then object B exerts a force on object A. This force is:
- equal in size
- opposite in direction
- the same type of force
But recently, I’ve been trying to move away from banning words and pushing my students to memorize dogmatic definitions, even if I think my definitions are better than what they’ll find in the standard textbook.
Instead, I’m trying to opt for us discovering more nuanced definitions together, and here’s one we came to with Newton’s 3rd law that emphasizes the fact that N3 pair forces are part of the same interaction, as seen in a system schema.
Now we instead emphasize how the two forces are part of the same interactions. What I just realized this year is how much easier it is for students to understand the idea of interactions rather than “N3 force pairs.” They intuitively get that an interaction requires two objects, and they easily see that there are different types of interactions-magnetic, contact, gravitational, etc. Of course, the really hard idea is that these interactions are symmetric—that the earth pulls on you as hard as you do on it, but that’s hard to understand from the N3 force pair idea as well.
Here’s how I think this will ultimately make a big difference in student understanding. A pretty typical question in intro physics classes goes like this “What is the N3 force pair to force X?”, as in “A ball is falling through the air, by Newton’s 3rd law, what force must be equal to the force of gravity?” (Of course, I would never say the force of gravity—it’s the gravitational force of the earth on the ball—but in my early days of teaching, I wrote exactly this question, and graded it wrong).
This question begs two problems—many students don’t know what Newton’s 3rd law is—egads. They often confuse it with Newton’s 1st law. But when I think about it, do I really care that they know these laws verbatium? No. Many students I teach are memorization vacuum cleaners—they can memorize even the original Latin of Newton’s 3rd law, and its translation, but this doesn’t put them any closer to understanding the question or the big idea of Newton’s 3rd law. I’d much rather students put these ideas in their own terms, and see the big idea that interactions are symmetric.
So I’ve rephrased that identify the N3 force pair question to go more like this.
For each force acting on the basket, identify the force that must be equal because it is part of the same interaction (Newton’s 3rd Law).
This gets at the big idea—do students understand that individual forces are part of an interaction, and can they identify these interactions? It doesn’t penalize them for confusing Newton’s 1st and 3rd laws, which is pretty much inconsequential to understanding the physics of the situation.
I think this also teaches more important ideas as well-there are hints here of how we work to define terms operationally, rather than using simply memorized statements. Students see that if we can state something more simply, we do. Forget all that ‘objects in motion will stay in motion’ stuff. How about—if Fnet = 0, the object will have a constant velocity, and vice versa. Most of all, I think students start to see that it is the process of science and scientists themselves that devises and refines these laws and definitions, not dogma, which is just one more step toward seeing science as a useful, approachable tool in their lives.