AKA Inventing a New Flavor of Energy Storage
When we start the energy unit, we usually pick up 4 flavors (kinetic, gravitational, spring, and thermal) and note that we will recognize and name other ways of storing energy when we see them. That happens pretty immediately when we do pie charts.
We run into a problem pretty quickly, here. The first way they try to resolve it is to make each pie entirely kinetic energy. Some students aren’t satisfied with this, but they might get convinced by their groups that there is no ∆Etherm because they just can’t see another way to make it work. Asking them how you can tell if a car was driven recently usually works. (“If you feel the hood, it will be wa… oh.”) So thermal energy must be increasing and kinetic energy must be staying constant. Something else must be getting smaller—can you drive a car at a constant speed forever? (“Sure, as long as the road keeps going and you don’t run out of… oh.”)
The gas must be storing energy somehow. And it’s not because of motion, a spring, a gravitational interaction, or warmth. So this must be a “new” way to store energy. In fuel. A chemical energy. I used to use Echem, but my students pointed out that chemical energy is an interaction energy, so it must be Uchem. Good point.
What does any of this have to do with tic tacs?
Right. So. At some point in the energy transfer unit, I will bring in some tic tacs and take a few minutes to do a quick qualitative problem with them. (Well, this is what I do now, but I’ve also used this as a quiz question in the past.) A funny thing happens when you drop a tic tac on a table. Sometimes it bounces higher on the second bounce than it does on a first bounce.
At this point, you’ll want to grab a tic tac and start dropping it. After you do it a few times, you’ll start to notice. It’s strange to see those “aberrant” bounces.
Last year, my students wanted to take a video to get a better view of what was happening. It took a few tries, but we got a pretty nice one. I suggest changing the quality to HD and watching full screen so that you can really see the difference in spinning.
These students have not studied rotational motion (and won’t study it in this class). We haven’t talked about rotational kinetic energy, but they recognize that there must be energy stored in spinning, so there must be a spinning flavor of energy. It’s a motion. It doesn’t depend on an interaction. So it makes sense for this to be a kinetic energy, not an interaction energy.
Here’s what that class ended up drawing to represent the situation. (I was trying out the TIPERs-style of energy bar charts instead of LOL diagrams. I’m planning to go back full time to LOLs in the future, but that’s a whole other story. In short: defining the system in the diagram itself is really worth it (for me).)
Another “new” flavor of energy to add to our list. It’s a pretty simple, quick, engaging, and physical-but-qualitative problem. And after seeing this problem, they start noticing other times when energy might be stored as rotational kinetic energy, adding a little more depth to the discussions of future problems.
Paulette Struckman who shared this phenomenon with me (and two of her students who used it for a science fair project) in my first year of teaching.
Andy Rundquist who has a blog post focusing on a totally different aspect of this kind of bouncing (with rugby balls, not tic tacs).