# Modeling Central Force: Day 1

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.