In the past, BFPM has been a bit of a morass for us—we’ve sometimes spent 4 weeks on all the stuff in this unit. This year, our goal is to clear it in less than 3 weeks, and hopefully closer to two. To do this, I’m trying not to get lost in whiteboarding every single problem, and trying not to feel a need to explore every detail the moment we introduce a concept.
In the past, this activity has taken more than 3 days to whiteboard out way through it. Today I wanted to get it done in 40 minutes.
So I gave the class 15 minutes to work on the Free Body Diagrams. I gave them some guidance to focus on being efficient—if they really got stuck on something, they should move on.
After 15 minutes, I passed out 12 1×1 whiteboards and asked each student to draw one of the solutions, intentionally including a mistake. I then put all the boards up in order in the front of the room, and asked students to go and draw correct FBDs above the incorrect ones, while also making a list of pro tips to help us draw FBDs in general.
After we got to near consensus, I walked past the revisions and said “I see some mistakes” and encouraged students to check out other boards. 2-3 minutes of thinking later and every board was correct. I was able to go through and pick out a few more pro tips like drawing in a dotted line for the surface, and highlight how the forces must sum up to zero in all the constant velocity problems and we were done.
Along the way, some students brought up some great questions, like whether the net force must act in the direction of motion of an object, and we were able to use these examples to develop an argument for why FBDs can really only tell us about changes in velocity.