Today in physics 11 I tried a new lab using our motion and force sensors, carts and tracks. The lab idea is from New Visions and I believe that the script that I was working from was written by Kelly O’Shea and Mark Schober.
I was pretty excited to give it a try because I’ve always just told my students that the area under a force-distance graph is work. With this lab, students develop the idea from direct evidence.
Students set up a tilted ramp such that they pull a cart up a delta h of 20 cm. The angle of incline can be anything, and this will play into the discussion after data is collected. The students pull the cart up the ramp and analyze their graphs. From their previous experience working with data and graphs, they look at average forces, trendlines, slopes of the trendline. After a short meeting where we try find a pattern, we all agree that there isn’t a strong pattern. I then challenge them to think of another way we analyze graphs and a few students said that we can look at the area under graph. That’s where we left it today.
Next class we will re-run the test and the area under the graphs should all be really close. I’ll then have them calculate the delta Eg for a cart that is raised 20 cm. From my own experiments, the results will be within 10%.
There were some problems with today’s lab though and I wish I could do a do–over. Overall it took the students almost 60 minutes to collect one run. Two groups did it in about 10 minutes, which is a lot closer to what I expected. The students were paralyzed by their data. Some groups had a negative slope in their trendline (their cart pull was slowing down), and that didn’t sit right with them. Other students were taking a very long time to plot a perfectly scaled sketch of their graph on the whiteboard, when the instructions were (and have been all year) to make a quick sketch. It was a bit tortuous and I showed some of my frustrations in class.
Looking back, I’m not sure if there is any advantage to not tell the students straight up that they should collect their data and analyze the area under the graph. I don’t any reason to have them first try and find a slope or whatever else they might try to find. I think I could frame it a bit different and allow for some exploratory time, but we didn’t need a full cycle of data collection that doesn’t deliver a useful result.
Another thing I could have tried is to show them an example graph and have a deeper conversation about why the graph looks like it does (accelerations and jumps in the pulling force). I did actually show them a graph from a demo pull but we didn’t discuss it in detail.
Overall I like the lab but that 60 minutes just about killed me, especially when a few groups finished in 10 minutes and we will need more time again next class. It was a sign that too many students didn’t understand the purpose of the experiment.