We tried Soccer Ball Inflation again this year.

I haven’t found many opportunities during our first semester of geometry for students to engage in multiple steps of the modeling cycle. So I’m glad for the few problems that at least let students define the problem, decide what information is useful to know, and begin to formulate a model to describe relationships between what is important.

We watched Nathan Kraft’s Soccer Ball Inflation video on 101 questions.

Most students wanted to know how many pumps it would take to fill the other balls.

What information do you need to know to figure it out?

This was the end of November. It’s not the last time I’ll ask my students what information they need to know to figure out the answer to a question, but it was the first. It takes practice figuring out what information is useful, especially when it has been given for so long. Most of what they wanted to know (except for the answer) isn’t very useful or even possible without complicated measurement tools.

So I asked, “What’s easy for us to know? What’s easy to measure?”

The radius.

Last year, I noted in my blog post that I gave them the circumferences (because that’s what Nathan included in Act 2, and I didn’t want to do any calculating). Dan called me out on this:

I’m not the only one who’s been living inside the “ideal” math world for too long. So have my students.

I asked, ”**Is** it easy to measure the radius?”

Oh. I guess not. The circumference.

Okay – so the circumference. I gave them the circumferences of all three balls. They knew from the video that it had taken 9 pumps for the smaller ball.

And finally … What assumptions are we making here?

They worked. I watched.

I’ve learned not to be surprised at the faulty proportional reasoning that happens every single year.

Most students said 14 pumps would fill the medium ball.

Why doesn’t that work?

The few who had gotten it correct actually calculated the radii from circumferences, and then calculated volumes from the radii.

No one recognized that the cube of the ratio of the circumferences would equal the ratio of the volumes.

And so the journey continues … trying to escape the “ideal” math world, one lesson at a time.