Day 32: Inertial Misconceptions


This picture is a student’s free-body diagram for a ball rising in the air after it was tossed.

Students are having a hard time shaking their “force in the direction of motion” intuition despite plenty of examples to the contrary.

WWBFD? (What Would Brian Frank Do?)

PS: The title of this post can be interpreted two different ways. Do you get it?


About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

3 responses to “Day 32: Inertial Misconceptions”

  1. jsb16 says :

    We spend some time taking about how I can’t push something after I’m no longer touching it. I mime trying to push a student/box/chair/hovercraft without making contact (a lot). And they’re restricted to only four subscripts for forces (for now): gravity, friction, push, and pull, each labeled with both source and object. Only gravity can exist between objects that aren’t touching.

  2. Frank Lee says :

    I’ve been having them do it very procedurally and systematically this year, similar to how jsb16 mentioned. I took a page from Randy Knight’s book (Five Easy Lessons…) and have them identify all the *contact forces* (normal, tensional, frictional, air resistance), then all the long range / field forces (just gravity for now). So far I’ve only had a couple students out of 100+ put "force of throw". However, I don’t claim to have fixed a misconception. It’s not that I changed their way of thinking, but I’m just preventing them from thinking. It’s intuition and thinking that "gets in the way" in this case. I used to be really into a more constructivist approach, but this year I’m thinking what happens if I just get them to do it right and maybe they’ll start buying it when they see the model works. This could possibly go against everything many of us believe, but it’s probably the way I myself learned it. I have as much evidence as my students that an object travels infinitely at constant velocity if forces are perfectly balanced. So who’s right? I dunno, but I know our model *works*. Until they witness their model breaking down, there’s nothing to stop them from thinking there’s a "force of throw".

  3. Frank Lee says :

    and btw, I know how horrible it sounds that I’m intentionally preventing my students from thinking.Anyway, I’ve mostly taken Knight’s procedure for identifying forces and slightly adopted it to my own needs. I had a number of students still severely lacking in this area, so I painstakingly wrote a little step-by-step guide. I have to admit I looked for a tutorial on drawing FBDs on KA, but well… I made this instead…

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