That was a moment.

There was a seemingly small, but actually big moment in honors physics today.  We putting our newly-developed model of universal gravitation through its paces, and started by looking at the Sun-Earth system.  After listing all of the stuff that we knew about the system (discussing fusion vs. combustion a bit, too), making a conscious decision to model the Earth's orbit as circular, and asking what we could figure out next, we decided to determine the Earth-Sun distance, given the Sun's mass.  It went the other way historically, I think, but this was good for our purposes.

The free-body diagrams started, followed by Fnets, and - before long - came the question/confused utterance (from a few sources in each section):

"There's a force acting on the Sun... ...and only one... ...but isn't it sitting still?"

The discussion afterwards about Newton's 2nd, approximations, etc. that got the problem train a'rolling is much less interesting than the train of thought that led to the question.

I made a big deal of it, because it was a big deal.  There was confusion because worlds were colliding - students have "known" for most of their lives that the Sun sits still and the Earth orbits it.  They have only recently begun to wrestle with the realities of Newton's laws, and, despite our best intentions and efforts, students can keep those two worlds mostly separate, mentally.  There's the "real world" and the "physics class world."  This is exactly what Eric Mazur is talking about.

Today, though, the confusion indicated that the two understandings weren't separate - they were both in there, trying to edge each other out of the ol' brain pan.  Having a gut reaction like that, seeing that it doesn't make sense for a single force to be acting on an object if that object's not accelerating, and not being willing/able to just push the "physics class understanding" to the side is exactly what my goal is for my students.

What a great present for a Wednesday!