First day of the semester yesterday. Wide-eyed undergrads were flocking to their physics classes in our brand new and completely beautiful building. I’m not so sure they were thrilled about it; doing physics is not something that naturally appeals to most denizens of the university. I admit I’m feeling a few pangs myself for entirely different reasons – though in a lot of ways teaching (and grading!) is a massive time sink, as a research assistant I do miss being in front of a blackboard and helping teach the next generation their physics. Maybe I’ll volunteer to do some tutoring now and then…
What are they learning this week? If I had to guess, probably linear motion in one dimension for the first class and static electric fields for the second class. Via Swans on Tea, we can see a great demonstration of one form of 1d motion. It’s high-speed photography by artist Alan Sailer of liquid-nitrogen-frozen objects being perforated with a pellet rifle. I’ll just sample one, you can view the rest at the link – this one is a plum meeting a pellet at some 200 meters per second:
There’s lots of freshman physics in this scenario. There’s finding the average acceleration of the pellet in the object by looking at the speed before and after. There’s finding the force the pellet exerts by using the acceleration and the mass of the pellet. There’s conservation of momentum in the way the fragments fly. There’s electrical currents in the flash that makes the photograph possible. There’s the energy and power involved in the light itself. There’s no end to the problems you can pose.
I propose we do an open-ended question to stretch our minds, since this one not a cut-and-dry numerical answer: why do frozen objects have a greater tendency to shatter than warm ones, even in the absence of an overt water -> ice phase change? Think about what temperature is on the atomic level, and you’ll be on the right track.