walk or take the elevator?

I'm just generally excited about getting back into the classroom after a long sabbatical. I'm thinking about problem-set problems for the Physics Majors. Here's what's in my head right now:

NYC has had a hot summer, with most buildings running air conditioning on a thermostat continuously. To save energy, NYU (and other large entities in NYC) asked their employees to conserve energy in various ways, some of which we might take issue with. Here's an uncontroversial one: You should take the stairs, not the elevator.

But is that uncontroversial? What considerations are required to figure out whether this policy would reduce or increase energy consumption? Obviously—if you take the stairs—you use less elevator energy, but then you drop a metabolic load on the building air-conditioning. Which uses more power in the end? Use a combination of web research and simple physical arguments to make cases, and identify weaknesses in your argument as you change assumptions. Things that matter include: Neither humans nor elevators are 100-percent efficient delivery vehicles for potential energy (in fact, can you see a fundamental argument that elevators must spend more than 50 percent of their energy generating heat?). Elevators are heavy but counter-weighted. Some buildings have very busy elevators, so your contribution to the elevator load is only the marginal contribution; in other buildings you are typically the only person in the elevator. Air conditioning systems have efficiencies limited by fundamental ideas in thermodynamics, but are probably much less efficient than the limits. And so on!

Thanks to Andrei Gruzinov (NYU) for starting me thinking about this one.

oscillations and the metric

In class, I was solving the normal-mode problem for a solid object near equilibrium, using generalized coordinates, in the usual manner. This starts by orthogonalizing the coordinates to make (what I call) the "mass tensor" (the tensor that comes in to the quadratic kinetic-energy term) proportional to (or identical to) the identity. This operation was annoying me: Why do we have to get explicit about the coordinates? The whole point is that the coordinates are general and we don't have to get specific about their form!

In my anger, I solved the problem without this orthogonalization. It turns out that this solution is easier! Of course it is: I can do everything with pure matrix operations.

I had two other in-class epiphanies about the problem. The first is that the solution you get when you don't do the orthogonalization is more analogous to the simple one-dimensional problem in every way. The second is that, in a D-dimensional problem with D generalized coordinates, the tensor that goes in to the kinetic energy term is some kind of spatial metric for a D-dimensional dynamical problem. (Or proportional to it, anyway.) That is simultaneously obvious and deep.

many-body systems; composite objects

Every time I teach mechanics (and this is something like the 21st year I have taught it at the undergraduate level) I learn something new. This week we are talking about many-body systems; I had two epiphanies (both trivial, but still): The first is that the description of the object in terms of a center-of-mass vector and then many difference vectors away from the center of mass (one per "atom") is purely a coordinate transform. Indeed, it is just generalized coordinate system that is related to the Newtonian coordinates by a holonomic transformation. Awesome! So when the Lagrangian separates into external and internal terms, this is just a result of the appropriateness of that transformation.

The second is that the definition of the many-body system is completely arbitrary. It should be chosen not on the grounds of being bound or solid or connected but rather on the grounds of whether choosing it that way simplifies the problem solution. Both of these realizations are simple and obvious, but it took a lot of teaching for me to get them fully. I am reminded as I realize these things that the physics concepts we expect first-year undergraduates to manipulate and be comfortable with are in fact pretty damned hard.