Day 20: Tapping, Net Tapping, and Lauren’s Law

Physics 10 started balanced forces today, and I tried a sequence inspired by Frank’s 180 posts. Thank you to Bowlmor for donating two bowling balls yesterday and to the art teacher for loaning us a rubber mallet (because Lowes only had one in stock). We started in the hallway with the bowling balls and some planning and testing for a few different cases (how to speed up, slow down, move at constant speed, and race without overshooting the end lines).

The toughest part by far was figuring out how to keep a ball rolling at a constant velocity. At first, they decided you needed to tap it occasionally. When they did that, though, one girl pointed out that “every time he hits it, it speeds up.” So then they kept wanting it to be tapped more and more gently and less and less frequently. Eventually, they thought it seemed like a pretty constant speed when it was rolled without tapping, but they thought it was tough to tell just looking at it.

I had made a few images using Clipstro on my iPhone when I brought the balls to school on Sunday, so when we returned to the classroom, we looked at those for more evidence. (These aren’t the best because I had to roll the ball myself, then run to get the camera in a good place and held still, but they were good enough for our purposes today.)

I was glad I let them look and sit for a minute, because even though they immediately agreed that it looked like a pretty constant velocity, they eventually followed up by saying, “that’s a lot of bowling balls.” Ah! They didn’t get what the image was showing them (even though before they looked at it, I had likened it to strobe images they see on TV during things like the Olympics). So I talked them through how it was made, and then I think they got it.

We had our tapping = changing velocity; no tapping = constant velocity rule set, then we moved over to the fancy fan carts and tried some things out. It was really neat, and they were good at coming up with other things to try without my even prompting them for that. Eventually, Lauren started talking about how if the tapping was even on both sides, the velocity didn’t change. But if the tapping was uneven, then it would change. We cleaned up the language a little and called it Lauren’s Law. Then the kids said, “No, Lauren’s first law.” Good point. Let’s plan for more discoveries.

Finally, we spent a few minutes thinking of English meanings for force, sharing them, and then getting my (no better, but more useful for physics) definition of force. Next class, we will pick up with naming some common types of forces and picking up new diagrams (system schema and FBD). Hopefully that won’t take the entire class and we will get to start on the problems and arguing with each other about balanced forces.