Teaching Newton's Third Law ("N3" from now on) can feel like walking through a minefield. Students come in with lots of misconceptions and they run deep. Constructing system schema is the best strategy I've found to force students to think about N3 the right way. The approach has revolutionized the way I teach this concept and I highly encourage you to embrace it!
"A book sits on a table, gravity pushes down on the book and the table pushes up. The force are balanced so the book doesn't move. That's Newton's Third Law!"
No. Just, no.Many students have this misunderstanding about Newton's Third Law and I've even encountered a lot of middle school teachers who have used examples similar to this to explain it. I know this is just anecdotal, but I think a lot of misunderstandings about N3 come from middle school teachers giving incorrect explanations. I don't blame them though! To be honest, I don't think N3 law is an appropriate concept for MS students because to really grasp it, you need to consider systems of objects. If I was a curriculum developer I would push to have middle school students focus only on single objects (with F=ma and the law of inertia). I would completely hold off on N3 until high school.
What are "actions" and "reactions!?" Where is the mention of forces!? What about objects. Does this definition imply that one force acts before another force? Does mean that there is a cause-effect relationship between forces in a pair? What the heck is going on!?
Despite its many problems, this statement seems to be one of the only things people remember from their HS physics class. Maybe I'm being dramatic, but I think that this sentence is the root cause of many, many , misunderstandings. It is a plague upon this Earth and must. be. eliminated. For the love of GOD, please don't use this with your students!(Unless of course, you're ripping it apart and explaining why it is, essentially, the embodiment of satan himself. )
