Class 2 – Tuesday February 1, 2011

From Maura:

We started by reviewing some of the homework problems.  Feedback I got:  the problems took a long time (absolutely!  that’s why we only assign a few problems); some students overthought the questions (that’s ok – one of the goals is to get you to think); some students weren’t sure where to start (also ok – practice will help here).

We spent the rest of the time on the carbon footprint graph from Chapter 1.  First we talked through the different words, with the idea that before we can get to the numbers we have to make sure we understand the vocabulary.  The internet can help here, or a good dictionary, or just talking to your neighbor.

Then we talked about the information that the graph presented, and discussed how the different activities resulted in carbon emissions.  As suggested in the text, the class split into groups of 3 or 4. Each group estimated the number of times each activity is done each day in the U.S.  I staggered the groups – the first one started with Google search, the second with movie download, etc. – and asked them to move on to the next activity when they finished their current activity.  Walking around, I was surprised to see so many students looking on the web for information.  Most groups saw that the U.S. population – estimated at 300 million – was a good starting point.  But I saw very specific searches (number of movie downloads, number of dishwashers, etc) that were unlikely to lead to the answers.

But there’s a very solid reason why the internet isn’t helpful here: it’s not what we’re asking students to do. One point of the exercise is to practice our Fermi problem skills.  That is, to practice making reasonable assumptions and putting them together correctly to get a ballpark estimate.  The internet – as compelling and as useful as it may seem – won’t do that for you.   We’re also giving students an opportunity to practice reasoning and problem solving, which are both important pieces of what we expect them to learn.  Again, the web won’t give you that.

As we worked in groups, the lab managers kept coming in to tell us that school was closing soon due to the bad snowfall.  So this put a bit of pressure to move along.  Even though the groups hadn’t gone through all of the activities, we put a few up on the board and talked about the estimates.  It was impressive to see that the Google search estimates were identical, and that some of the other estimates were in the same ballpark (or “order of magnitude” if you want to be technical).  Other estimates were very far apart – in one case, that was because the students had calculated the number over a longer period of time (a year, I think).  The most difficult one was movie downloads.  One group said they found a site that claimed that there are 5 million NetFlix downloads in one day.  That seems awfully large to me.  Another group looked at iTunes downloads and used the to estimate the number of movie downloads.  And another group wanted to talk about illegal downloads.  It’s very easy to get distracted into different issues when doing these problems!  Again, the idea is to make some reasonable assumptions (if you want to include illegal downloads, that’s ok – but state it clearly) and do the calculations from there.

We didn’t get to go through all of the estimates because of the distractions from the snow.  We’ll revisit this at the next class and also talk about the metric prefixes, which is a good introduction to Chapter 2 on Units.

From Ethan:

SNOW DAY (school closed at 12:30, right after Maura’s class, right before mine). I will resist the temptation to make up snow day Fermi problems.


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