The 10th grade class finished whiteboarding the big set of FBD practice problems today. The last problem was a challenge problem I got from Frank with a book being held against a wall by a log. We didn’t use the hairbrushes, but I pushed two PAScars against my folder and they immediately said, “oh, there’s friction!”. Then they asked me to put one wheel-side and one not-wheel-side against the folder so they could see if it was possible for just one of the objects to be exerting friction on the book (yes)—but then decided it was likely there was friction with both surfaces anyway.
They are still sorting out the difference in ideas between balanced forces meaning all forces are the same size and balanced forces meaning the forces add to zero. So once they had that FBD, they were worried it couldn’t be balanced (which they knew it needed to be since the book has a constant velocity).
They sorted this out pretty quickly though, realizing that either Fg must be larger or the friction forces must be smaller. (I forgot to get a final photo).