Starting this post before class.
This graph was in today’s Globe:
I think it has lots of Fermi problem potential. I’d like the class to rank the categories in order of total CO2 footprint. That means estimating the total number of google searches, movie downloads, …
I will ask them to work in groups of two or three, by row.
I will also give back the exam.
Now off to teach.
I think the class went really well. (I think the students thought so too.)
We started with a fair amount of vocabulary/general knowledge. Many didn’t know much more about global warming than that it was in the headlines. So we talked about that, about how greenhouse gases (CO2) trapped heat, about why increasing CO2 concentration might be (and was thought by most knowledgable scientists to be) man made – the result of releasing the carbon stored in fossil fuels (coal and oil). The idea of a carbon footprint was new to all of them (I think). They enjoyed speculating about what contributed to the carbon footprint of each of the activities in the chart.
Then it was time for the numbers. I wanted to figure out which of the activities had the largest total carbon footprint. That meant estimating the number of google searches, movie downloads, daily newspapers printed, glasses of orange juice, dishwasher loads, five mile drives and cheeseburgers. We decided that we would find the counts per day, in the United States.
The students sit three to a table in front of computers, so I used the seating to divide the class into teams of three (occasionally two). Each team started with one of the seven estimates and, when done, moved on to the next, circling back to google search if they reached the end of the list. I started the teams in different places, so that when we were done (after about half an hour) we had two or three estimates for each of the seven tasks.
There was a buzz in the room while the work was going on. Dori (the tutor) and I circulated, answering questions as needed. I found that often what I needed to do was to give permission to simplify, approximate, round. Sometimes the students were trying for more precision than was necessary or possible. I also needed to remind them that these were team efforts, not three individuals tackling each problem. (If there’d been just one computer on each table that would have been clearer. As it was, many teams started their work with independent searches.)
I called a halt to work with about 20 minutes remaining in the class, so that we could put the results up for all to see. I think the students were surprised to see that groups working independently used different methods to arrive at essentially the same estimates. That taught them that there really might be a correct ballpark estimate – something I think they doubted when they just watched me do Fermi problems or did them one at a time themselves.
I won’t put our conclusions down here – do the estimates yourself. I will put this problem in the growing text of Common Sense, and will suggest there and in the instructor’s manual that it makes a wonderful class.
I did spend a few minutes at the start of the class handing back the first exam and commenting on the scores. In each exam I wrote a short note with a letter grade so far for the semester (not just a % score for this exam) , along with suggestions on how s/he might improve.
I drew this histogram on the board, and remarked on how few students had taken advantage of the opportunity to redo the exam at home over the weekend to improve their grade.
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