On some days, being a physics professor is a real slog — many-hour meetings, struggles with grant funding, endless emails, and near-futile attempts to eke out time for actually thinking about science. But there are other days in which it’s really awesome. Today, for example, I got to cart around an elephant femur. Here it is in my office, with the kids for scale:
Why do I have an elephant femur in my office? In my “Physics of Life Class” (i.e. biophysics for non-science majors; description here), we’re discussing biomechanics and bone size — why big animals need disproportionately wide bones compared to smaller ones. I’ve illustrated this before with pictures of animal skeletons, but I learned recently that we have on campus an actual elephant skeleton. The elephant was named Tusko. He worked in a circus about a hundred years ago, and had a sad life — he’s been referred to as “the world’s most chain-bound elephant.” To learn more about Tusko and how he posthumously ended up at Oregon, see http://cas.uoregon.edu/2014/02/the-elephant-in-the-room/. Thanks to Edward Davis, I was able to borrow Tusko’s femur, and cart it across campus to class. Thanks to Samantha Hopkins, I also had a dog femur.
One gets a lot of stares pushing a cart with an elephant femur. Random students:
Kyle Lynch-Karup, a teaching assistant for the course:
A brief summary of the physics: The elephant’s femur isn’t just a proportionately larger version of the dog’s. It’s about 10 times longer, but nearly 20 times wider. Why? Leg bones have to support the weight of the animal, which is proportional to its volume, which scales as length to the third power. (Think of a cube: it’s volume equals the length of a side, cubed.) The strength of a bone, however, is proportional to its cross-sectional area, which scales as length-squared. (Think of a square.) So as we imagine enlarging a small animal, its weight increases much more than its bone strength, if we keep its proportions the same. To counteract this, large animals have disproportionately wide (and hence large-area) bones. For the experts: the really neat thing is that behaviorally similar animals like antelope, wildebeest, etc., have bone diameters that scale as length^1.5, exactly the form for which bone strength and weight have the same scaling with length. We worked through all this in class; the very general message of a lot of what we do is that scaling arguments are powerful.
I’m increasingly fond of having students work through worksheets in class, in small groups as I wander around commenting and helping; here’s today’s: Bones_allometry_and_mechanical_similarity It works much better than lecturing!
So: I got to play with an elephant femur and teach people about allometric scaling. I also didn’t have any meetings, I went to a neat session of our science teaching journal club, and I attended an interesting seminar on quantum measurement. Today was a good day. I didn’t get to work on an image analysis puzzle I’ve been dying to spend time on, but that’s why I’m quoting Ice Cube rather than Lou Reed in the post title.
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