The force diagrams I came up with have "invaginated arrows".

In the std diagram students get confused about what forces act on which objects so the invaginated arrows trip them up just enough to make them figure out the right orientation on their own ..

If you press me, I'd say these are nothing more than UX trials..

I support that statement! Where do you think UX designers learn about A/B trails in the first place?

https://www.open.edu/openlearncreate/mod/oucontent/view.php?...