*Note: Since writing this post I have significantly changed how I start this unit; see new post here. *

Today, which happened to be Pi day, we started circular motion. Too perfect. I wanted to start the unit by building a model for centripetal acceleration, similar to the way I started my momentum unit. We had briefly (for a couple of minutes) looked at circular motion when we built the balanced force particle model, the methods for which I stole from Kelly O’Shea (the Kick Dis puck is amazing for this! Seriously check out her post on BFPM). But that was quite a while ago.

I started class by telling them that we were going to investigate circular motion, specifically the snapshot at the bottom of a vertical circle. I told them how to set it up, showed them how to vary the height so the speed is varied (and had a side-note conversation about why the height is the only thing that can change the speed at the bottom; gravity is the only force doing work), and showed them how the photogates will take a velocity measurement using the diameter of the mass. I had each person find a partner (I group them for formal labs but have been generally letting them form their own groups for modeling, that will change soon just to switch things up), and then I had the pairs find a ‘cluster’ as well. The idea is that each cluster would have a constant radius but different masses. Then I cut them loose.

One cool idea a student came up with almost right away was that if you zero the force detector with the hanging mass on it, it reads the net force. I was planning on having them subtract the weight of the hanging mass after zeroing with nothing on the detector; how awesome that a kid figured out a much better way to do it! Another student showed me how to Use a smartphone for the diameter of the pendulum mass with the smart ruler app, picture at right. I love learning from students!

**Pairs then clusters**

One thing I did that I liked for this round of modeling was the idea that students have a partner with which they collect data, then a ‘cluster’ of 3-4 pairs to compare data. Each cluster had a constant radius but different masses, so they could investigate the effect that mass has when the radius is constant. Then I had the clusters mix to look at the effect of same mass with different radius, but more on that later.

**Quotes and moments**

“It’s *really* entertaining watching the force in real time. It’s like someone’s having a heart attack!”

At one point I tried to engage one pair in conversation about something trivial. They ignored me so they could keep taking data. It was a proud moment for me as an educator.

“It’s something different!”

**Wrapping up**

Today was mainly a data collection day, but most groups got the the point of plotting F vs. v to see that it is quadratic, and this piece of the data worked out fairly well. Nice quadratic graphs for most groups, which was certainly a good start. Tomorrow we look for patterns.

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