## Soccer Ball Inflation

13 Dec

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?”

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.

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.

1 Comment

Posted by on December 13, 2015 in Dilations, Geometry

### One response to “Soccer Ball Inflation”

1. December 13, 2015 at 7:58 pm

Again you get me thinking.
Interesting that no-one considered the fact that air is compressible, and so making it difficult to be precise about the number of pumps to fill the ball.
With some actual balls and a real pump you could let them investigate bounceabilty against number of pumps. Also measure the volume of the pump to explore ideas of pressure.
A separation of math from physical phenomena has apparently developed over the years, leaving math out on a wing and physics struggling to explain stuff. One result, for example, is the second video in the Graphing Stories so beloved by Dan Meyer.

I came across this the other day:
“There are 100 cookies on the plate. Every 5 minutes the students eat 5% of the cookies. How long until half the cookies are eaten?”. Reality check required here !!!!!!!