by dkarl | 2025-04-01 17:50:37+00:00
At my high school, somehow physics was the dumb jock science course. I think it was because the head football coach taught physics for decades before retiring my sophomore year. Anyway, as a kid who was doing well in school and was headed for college, it was a natural decision for me to not bother taking physics and study for the AP test on my own. But one day a kid showed up in one of my classes with a hall pass for me to go to the physics classroom. The new teacher needed my help.

She had planned on teaching a lab on gravity and acceleration that day, but she was having trouble getting the right experimental results. Now, this story is not going to reflect well on her, so I want to say up front that she was already taking physics education at my high school to unprecedented heights by 1) trying out the lab on her own before trying to teach it, and 2) actually giving a shit about the results. I doubt the coach who had previously taught physics ever bothered to do any of the experiments himself, and I'm guessing everyone who ever turned in a lab report to him got an A regardless of the contents.

So there I am, a future physics major walking into a physics classroom for the first time in my academic career. I'm nervous because I have a reputation as a smart kid, and specifically as a smart science and math kid, but I was better with math and theory than with machines and measurements. I'm excited about getting to look smart in front of the other kids, but I'm also sweating bullets that there might be something about the equipment that I might not be able to figure out. So I ask her to show me what the experiment is and how she's doing it.

The experimental setup is a small but heavy piece of metal attached to a long, thin strip of the kind of paper used for carbon copies. (Or carbonless copies maybe. You know the paper where you write on one sheet, and there's a pressure-sensitive sheet underneath that creates a copy? It was a long strip of that pressure-sensitive paper.) The final piece of the experimental setup was a loud clacking thing that the strip of paper fed through. When it was turned on, a little hammer inside it slammed down every 1/4 of a second. The idea was, as the paper traveled through, the hammer left a mark every 1/4 of a second, and you could measure how far the paper traveled in each interval between the hammer strikes. Much more precise than a stopwatch!

You have already figured out how the experiment works. You hold the clacker at a fixed height against the wall or some other high fixed point, thread the weight end of the paper through it, turn the clacker on, drop the weight, and the clacker leaves marks on the paper that let you calculate g.

The teacher understood this, to an extent. But she decided that it would be less of a logistical hassle if the students did the experiment at their lab tables, by holding the clacker on the table and pulling the weight horizontally across the table with their hand. She tried this quite a few times herself, plotted the numbers, and could not get the plot to look like a parabola like in the textbook. I explained to her, "We're measuring gravity, so gravity has to do the work. If we move it with our hands, we're just measuring our hands. If gravity moves it, we'll measure gravity." We tried it, it worked, and she sent me back to whatever class I had been in when she sent for me.