# Day 58: How We Roll

Monday, Dec 3

Physics: Still working on universal gravitation and intro to cosmology. Today we watched another excerpt from Steven Hawking’s Universe.

Honors Physics: First introduction to wave superposition. I spent a considerable time discussing phase now. Usually I kinda hand-wave that and simplify to “in phase” or “out of phase” and talk mostly conceptually, but with this class I decided to go a little more deeply into the connection to trig and put some numbers on phase and reviewed values for the multiples of 90 and 45 degrees. A student asked the perfect question about how waves interact and if it just happens at one place at one time or what? Beautiful segue into circular wave interference, where we’ll pick it up again tomorrow.

AP Physics: It’s time to do the rotational inertia.

We started with some rotational inertia sticks and body movements to get a feel for the idea before getting into the math of it. I’m going to make them memorize the I coefficient for a variety of shapes and the parallel axis theorem this year. And assess the hell out of it. In the past I’ve let that slide a bit, but everyone’s rotational motion life is so much easier with those pieces of knowledge at their fingertips. The other goal was to do an Atwood’s machine problem where the pulley has some mass and show the similarities/differences with the assumption of no mass.

I’ve been reading a book that has a cog-sci- and evidence-based learning methods, and one of them was looking at contrasting cases that are exactly the right amount different to highlight the important things between them. I gave that a try today with the Atwood’s and thought it went well. It was interesting to show them how the linear analysis skills are still super important, and the only difference on that end is you can’t assume the tension is equal throughout the rope any longer. Then you add in all the rotational analysis, sure, and have to then connect rotational and linear motion, but that is incredibly simple now that you have a reason to use it compared to it being a rather pointless exercise in algebraic frustration and variable management when it’s practiced before.