Archive for September, 2008

Collection of Traffic-Related Science

In my last post about the 35W bridge and the traffic problems it caused, I promised that I would look for some traffic-related science.  So here’s a collection of links I’ve found, plus some memories of older stuff that I’ve read that I haven’t taken the time to track down.

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A Bridge to Somewhere

On August 1st, 2008, disaster struck the Twin Cities as the Interstate 35W bridge that spans the Mississippi River between Downtown Minneapolis and the north side of the city collapsed during rush hour.  Thirteen people died and 145 were injured.

The aftermath of the Collapse

The aftermath of the Collapse

I did not live in the area at the time, so I can’t quite share the same feelings that must have been shared by everyone in Minneapolis-St. Paul.  I’m sure that most people either used the bridge regularly, or know someone who did, and the thought “That could have been me or someone I love” must have been pervasive.

The effect of the bridge collapse on my life has been much smaller.  With one of the city’s main arteries cut off, the effect on traffic throughout downtown and the surrounding area has been huge.  This is especially pertinent to me, since I live near downtown and have to get to the East Bank every day to get to the University of Minnesota.  Since bicycling is my main mode of transportation, increased traffic is obviously an issue, although the bike lanes have made this fairly manageable.  The other problem is that I have to find a way to get across the river on one of about five bridges, which are of course carrying the load that the interstate bridge used to take.  In addition, the bike trail along the river bank has been cut off where it passes underneath the bridge, further limiting my options.

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Grading the First Quiz

The forecast for rain in the morning followed by sunshine turned into rain in the morning followed by more rain and clouds, so I decided today would be a good day to tackle my first grading assignment as a TA.  The physics for biological sciences class had their first quiz, and I was scheduled to grade one of the four problems.  And after four hours of work, they are all graded, and the grades have been reported.

I’ve gotta say, I’m glad that I graded one of the problems on the first quiz, because I have a feeling that easier problems are easier to grade.  The easiest grades to assign are “perfect” and “zero,” because you usually don’t have to spend much time trying to decipher what the student meant to write, what they were trying to do, how close that is to a correct approach, and finally, how many points that work is worth.  Since this was a pretty straightforward unit conversion, there were lots of perfect responses, which I’m sure greatly reduced the amount of time I spent on the grading.

My method was to quickly go over each paper, sorting them into piles that roughly correspond to grades A-F.  Since there were about 200 papers, this took me about an hour, for an average of about 18 seconds per paper.  The first checkpoint was getting the right numerical answer for both parts (which would have taken me even less time if students knew how to box their answers!  I’m not mad, though.  I’m not bitter).  Students with two right answers go in the A pile, students who only answered one part go in the D or F pile, and I have to look a little more closely to separate the B’s and the C’s.

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Assume a Spherical Physicist

In physics, it is often necessary to make certain assumptions to simplify a complicated problem to make it tractable.  We might make assumptions about symmetry, say that a certain small value is essentially zero, talk about what happens when a certain value is infinity, or any of a host of other simplifications that would make a mathematician cringe and make a layman wonder how he can take our results seriously.

We often make light of this tendency by talking about spherical cows.  Obviously, a spherical cow is a pretty ridiculous picture:

A cow only a physicist could love.

A cow only a physicist could love.

However, in certain situations, estimating the cow as a sphere with a characteristic radius might not be as ridiculous as it seems.  For example, if the car were flying through the air (or standing in a strong wind, if flying spherical cows are too much for you to accept), the air resistance on a sphere the size of a cow would be a pretty accurate approximation.

But of course, the spherical cow is most useful as a metaphor for the approximation techniques we do use.  So what are these simplificaton techniques?  Read on to find out.

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Friday Afternoon Poll: Quantum Mechanics vs. Relativity

It’s Friday afternoon, and after a day full of classes, I have an hour gap between the introduction to research seminar and my dreaded TA team meeting.  Since my brain is in the final stages of shutdown, I figrued it would be a good time for a fun, somewhat nonsensical poll.  So here we go:

If you could change the laws of physics, which would you get rid of: quantum mechanics or relativity?  And I don’t mean this esoterically, like finding a quantum theory of gravity to explain relativity, or something like that.  I mean you say “BAM!  Now the wave-particle duality no longer exists!” or “WHAMMO!  No more time dilation!”  And your choice would immediately affect all physical processes from that point forward.

There are no cop-outs allowed.  You can’t say that you would keep both.  Put your answer in the comments section, along with some sort of explanation for why you would choose to do it that way.

Since I just came up with this question not 15 minutes ago, I don’t have an answer myself yet, but I’ll let it percolate through my half-functioning brain and let you know what I decide.

Beam Day for the LHC!

Today is the day!  The first beam of protons in the Large Hadron Collider will be circulated today at about 2 AM Central Time, September 10th.  This is a big milestone for high energy physics, and you can find information about it all over the blogosphere and the internet at large.  I recommend that you check here for Cosmic Variance’s liveblog, and the CERN website for information on the startup, as well as a live webcast.  Not sure exactly what they’ll be showing on the webcast, but if your an insomniac (or a European), and you’re looking for some physics-related entertainment (and who isn’t?), it should be worth checking out.

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First Week of Class

So I’ve got the first week of grad classes under my belt.  Not that I had expected a huge difference from undergrad, but the classes aren’t qualitatively different.  Of course, there are significantly more students in my classes than at my small undergrad program where I had six people in my classes.

Taking notes

I’ve been struggling somewhat with the chronic problem of physics classes: Do you try to really understand what the professor is doing during the lecture, or do you try to copy down everything, hoping that you’ll understand it later?

Of course, in principle I should know, for example, where Lagrange’s equations come from and how to derive them, but I also know from experience that I probably won’t be required to reproduce this in homework or exams, but rather how to use them to solve problems.  And when the professor is tossing around all kinds of cross products and indices, and arguing why this or that term equals zero, it’s very easy to miss the forest for the trees.  I’d say that in many traditionally taught classes, it’s pretty much impossible to both understand what’s going on and to get everything in your notes.  Most students tend to opt for the latter, hence the addage that “Lecture is the place where the class notes are transferred from the notebook of the professor to the notebook of the students without passing through the brains of either.”

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