ABCs of FBDs

This year marks the fourth I am teaching some form of introductory physics. Some factors have changed (institution, textbooks, student ability level) but I’m finding that some things remain the same. I’m getting a better handle on the common misconceptions my students have.

A big one revealed itself this week regarding free body diagrams (FBDs) and Newton’s third law (or N3, as I abbreviate it.) Randall Knight’s College Physics textbook, which we’re using, spends a whole chapter on just understanding what forces are and practicing drawing FBDs. I appreciate this approach, especially for intro physics for non-majors, because it helps students face their misconceptions about what forces are and helps them understand that the drawing of the scenario often motivates the solution.

This Wednesday in class I broke out for the first time this semester our mini-whiteboards,* passed out markers, and got the students into groups of three. We practiced drawing FBDs straight from drawings and translating FBDs from word problems. Each group switched drawers for each successive problem so that everyone had some practice while the other members of the group advised.

This procedure was really revealing: FBDs AND action-reaction pairs were all over the place. Below is an example of what I’d draw if I were working one of these problems (Rachael and Jon are pushing on a box, which is moving at a constant speed in the direction Rachael is pushing), and what the students typically drew (sans hands actually gripping a box, feet with toes on the floor; they got quite detailed with their drawings!). Note that the vector lengths are not in their proper scale in either drawing (they should be; I was just a bit lazy in drawing this for this post.)

Note that the students have drawn the action-reaction pairs for all the forces, instead of just the ones acting on the box. As they were drawing, questions started popping up, largely along the lines of  ”if everything has an action-reaction pair, how does any movement happen at all? Isn’t everything just in a deadlock?”

What to take away from this? First of all, getting students to draw in class and getting a look at their work can be very revealing. If I were to have just drawn the figures for them in class and they copied along, we would not have had this chance to confront the misconception. They might have gone on from this point just doing their drawings “because Dr. Whitney drew them this way” and not understanding the utility of a FBD. They might not have even gotten the drawings right from this point on, which would then affect the work they would do as we approach future topics like torque.

Secondly, I’m not sure I’ve ever seen a textbook that clearly articulates that identifying action-reaction pairs for N3 and drawing FBDs are separate analyses (that sometimes work together)  with different purposes. Admittedly, my sample size is small; when I was in college we used Serway. I once taught as an adjunct using Young/Freeman. My first year of college teaching we used Giambattista and I switched to Knight the next year. Knight does go through the practice of drawing two FBDs for two objects and then drawing a connecting line between the action-reaction pairs, an approach I appreciate. But not all problems use two objects with clearly identifiable N3 pairs. Sometimes we’re just interested in one object sitting on an inclined plane. The weight does of course have an action-reaction pair (with the earth), but we don’t draw it in. And so when students are left with something not having an obvious N3 pair they are sometimes  conflicted about what to do.

From this situation, takeaways for me are to (1) get the students to be active in class, (2) give yourself opportunities to see their minds working so that you can facilitate corrections, and (3) always be on the lookout for ways of knowing that are obvious to physicists but should be stated clearly for students.

Preview for my next post: I’ve got a strong policy of reading the textbook very deeply and carefully before class begins. Why did my students draw such complicated drawings of FBDs, instead of in the simplified form as directed in the textbook?

*Our intro room has been designed to have whiteboards all around the classroom to facilitate work like this, but because enrollment has grown in my class we’re meeting in a room in a different department.