Day 38: Omega = butt

Thursday, Oct 26

Physics: We went over the lab and talked about why the tension measured by the scale attached to the meter stick was greater than the calculated centripetal force (grr argh). They’re still not super clear on how vector components work, but they are really good at right triangles, so hopefully I can merge those two ideas together even more as we go on.

To talk about tangential velocity vs angular velocity, we went outside and stood in a line holding hands with me at one end. I instructed them to keep the line even with my shoulders as I turned around, and they very quickly discovered that the further they are away, the faster they had to go. Let me tell you, this was not popular to make them do at 8:20am on a Thursday, but I just let them grumble and had them do it anyway. Then we swapped out the people on the end and moved them to the beginning for a few more sweeps so people could experience multiple positions within the line. After coming back inside we went over the math that relates angular velocity and tangential velocity. I demanded that they make their omegas look like butts, because I’m tired of weird, sloppy ‘w’s.

Honors Physics: First I had them do AP B 2005 Q2 as a formative quiz to see how they’re doing on pendulum problems. There’s a lot of confusion about when to set the sum of forces to zero and when to set it to v^2/R, so we’ll be doing much more practice with that. After that, we moved on to looking at banked curves. All of it was symbolic problem solving, and tomorrow I’ll introduce questions about how changing one variable would change the other aspects of a banked curve situation.

Someone on Twitter asked me if I really went in the order of vertical circles, a conical pendulum, and then horizontal circles. Yep! Because we started with a pendulum. Vertical circles and a conical pendulum are variations on a regular pendulum in terms of analysis (only tension and gravity acting on the bob), so they all go together nicely. Horizontal circles can have any force acting on the bob making it go around (friction, gravity, tension, normal, spring, all of them) which makes the overall analysis more complicated. A pendulum, vertical circles, and a conical pendulum reduce the number of interacting objects and let the students focus on how the object is moving instead of having to deal with multiple types of forces and solving more complex systems of equations. They’re still not clear on when to set acceleration equal to what, so simplifying the FBDs is helpful.

I approach all of this content in terms of having four basic routes of analysis: kinematics, force analysis, work/energy/conservation, and momentum/impulse/conservation. Stuff moving in circles is just a force analysis. When we get to rotational, it will still be those four routes of analysis, but looking at a different type of motion. Even all oscillations use the same techniques. That’s why I avoid talking about “centripetal forces” or “centripetal acceleration”. I just have them analyze the forces the same way we always have, and if stuff is moving in a circle you deal with that in the “analyze the acceleration” step, not from the very beginning. FBD, sum the forces, set it =ma, figure out whether your acceleration is zero/linear/making things move in circles, substitute appropriately, and then solve. It’s that last step where the students get confused, but that’s why I approached it this way, so that they can do the majority of the process, and we can hammer on that last step more and more as we go.

AP Physics: Pig lab! Before the lab they took a quiz that assessed them on one thing I cared about and graded plus three things I cared about but did not grade and just wanted to see how well they could do based on what we’ve learned so far. This worked out so, so well. They all got full credit on the part I wanted, but I was able to give a lot of feedback on the other parts we’re just learning now and see where they are. I know formative assessment has been a thing for a long time, but this particular method of only grading half a quiz but giving feedback on the whole thing had never occurred to me before.

This is a terrible picture of the pig, but shows the basic set-up. It was pretty garbage and caused a lot of problems, but it was the best I could put together with what we had on hand before the lab on Tuesday, and I forgot to put something better together before today. I already have a set of fishing lure swivels in my cart on Amazon and will add “flying pig stands” to my list of things to get when I update the equipment.


At least my pig hat was a big hit.

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