# Day 52: Lab Planning & HR Diagrams

AP Physics: Lab Planning

Today, we started working on a lab to find how radius and mass affect the rotational inertia of a “T”. Due to some equipment conflicts, we’re using meter sticks and stopwatches, which means lots of calculations to get from the time it takes the mass to fall to the rotational inertia of the T, so we took today just to plan. I had students start by working backwards from the rotational inertia to the quantities they could measure for the falling mass and set up equations so they can just plug and chug during data collection. With finals next week, it worked out really nicely for students to have to go back to constant acceleration and unbalanced forces to make sense of the lab.

Earth Science: H-R Diagrams

Today, we focused on interpreting the H-R plots students made yesterday. I started by having students group the stars on their plot by type (giant, super giant, and main sequence), but held off on giving the names. I had students compare the giant stars to the main sequence, to see that the giants are brighter for a given temperature and asked them to come up with some hypotheses for why that is. This turned out to be much harder than I expected; I think the biggest problem is my students weren’t connecting absolute magnitude to any physical reality. Next year, I want to think about a bridging activity between the light intensity lab and this one to give magnitude some physical reality. It may also help to spend some time on the connection between the size of a light source and its magnitude.