I've heard all sorts of bright ideas about teaching Newton's Third Law. I've even tried a few myself.
One gentleman, whom I saw at a Florida AAPT meeting back in 1997, actually brought out a wooden model of a pair -- the fruity kind. He explained that "forces come in pairs..." and opened up the pair to show, inside, two force vectors labeled F (A on B) and -F (B on A).
That's clever and laugh provoking, but not particularly useful. It's not getting students to remember Newton's Third Law that's tough -- it's getting them to use it correctly. Me, I tend to shy away from the videos, from the cutesy pictures of faces being punched (and delivering a punch to the fist!), and so on. But I certainly don't have a magic formula for teaching this simultaneously easiest and toughest of laws.
As an example of the uphill battle we face teaching the third law, I'll tell a story of my own inadequacy. I'd love some advice.
I taught the third law this year to my general physics class. In the past I've restricted the third law to my AP sections, but I decided to give it a go... and that go turned into a full stop.
My classroom presentation was, I thought, strong and straightforward. I wrote Newton's Third Law on the board as "the force of A on B equals the force of B on A." In discussion with the class, we emphasized the difference between an object experiencing a force and an object applying a force.
One useful technique I have discovered is to insist that all forces be described in what I call "Newton's Third Law Language" before approaching a question. When asked for the third law pair of the "normal force," students are up a creek; but when asked for the third law pair to the "force of the table on the block" they can handle themselves.
So in class we practiced describing forces in Newton's Third Law Language, and then finding the third law pair. I used check-your-neighbor questions, I called on some folks randomly, I led a discussion, answered questions... all the usual techniques to make physics lecture effective. I even specifically addressed the companion forces to an object's weight, and to a normal force.
(The one thing I didn't do is the quantitative demonstration -- that's coming shortly. I'm attaching two force probes to two different-mass carts, and showing that the probes read the same thing when the carts collide with each other.)
On the nightly homework, I began with a conceptual question: Can a Newton's Third Law force pair act on the same object? Everyone got this right. Good start, I thought.
The next homework question asked to identify two equal and opposite forces that were NOT third law pairs. Students struggled with this one -- many said that such a thing cannot exist. Maybe I should have asked that question later.
Finally, I showed a kid dragging a sled across a rough surface. I asked for the Newton's Third Law companion force to the normal force, the weight, the tension in the string, and friction. Remember, we had done two of these THE SAME DAY, IN CLASS.
Do you know, only one person out of 22 got all of these correct? Well, I know. Their performance stunk. They all said that the normal force's companion was the weight; that the tension's companion was friction. BOUX!!!!!
I have confidence that, after enough iterations, most of my class will eventually be able to answer Newton's Third Law questions correctly. I know that I have to be patient, I have to make the class confront their misconceptions by getting problems like this wrong, I have to ask the same questions in as many different ways as possible.
Nevertheless, I ask the reading audience: has anyone had success teaching Newton's Third Law quickly?
(Picture at the top from The Physics Classroom. Visit this site -- it's good.)