## Wednesday, November 29, 2017

### Can you feel the inertia?

I just started rotation in my AP Physics C class and I introduced moment of inertia today. There was the traditional hoop vs disk down a ramp and I found an inertia stick for students to observe. The first class went very poorly because I introduced the rolling inertia demonstrations before we had talked about rotational kinetic energy and rolling ... BUT I did develop a new demo that did work so let's focus on that.

My textbook has a lot of pictures like this one of unusually shaped objects:
The odd objects are meant to illustrate that the moment of inertia is different with different axis positions. This concept will also help explain the Parallel Axis Theorem if the object is not one of the uniform shapes commonly seen on a moment of inertia chart. I wanted to make something like this with a movable axis so that my students can feel the difference in inertia at different axis points. Its a good thing I have a mini-shop in my prep room ...

I found a scrap piece of 3/4" plywood and used a bandsaw to cut it into an "odd shape." Then I drilled three 1/2" holes through it, one on each end and one roughly at the center of mass. I cut three 8" pieces of 1/2" dowel, one for each hole. The dowel was snug enough that when you rotate it, the whole piece rotates.
The wood is hefty enough that you really feel a difference from the center axis to one on the end. You can feel a slight difference between the two ends but not as much as I'd like. I may carve out a bit more so that the difference is easier to feel. Below is a video of me spinning it to give you a sense of it.