Gas station without pumps

2013 August 4

Units in physics again

Filed under: Uncategorized — gasstationwithoutpumps @ 17:22
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Joe Redish has a series of posts on The Unabashed Academic on the use of units in math and physics courses: Units and stoichiometry, Cutting mathematicians some slackTeaching units, and A higher power—units again.  I just re-read that series of articles today (having read one or more of them when they first came out).

The Teaching units post had one paragraph that particularly caught my attention:

In addition to suppressing units when we do pure math, we might also note that when writing computer code we often suppress units. (This led to a multi-million dollar failure at NASA.) But any object-oriented programming language can correctly retain (and even check) units. I doubt that classes in computation, either in computer science, engineering, or physics departments, teach objects with units—despite the fact that it would be incredibly useful for scientists.

I used Vpython with my son in learning calculus-based physics, and was irritated by its inability to play nicely with units (see Units in physics and Physics simulations). It would be a fairly simple modification to Vpython to allow a “units” option for each plotting or drawing routine  There could be a global option to change the defaults from None (for legacy Vpython code) to “m” for lengths, “s” for time, and “Hz” for frequencies (like the “rate” function), and to require units using the Unum package for all plotting and drawing.

For teaching physics, having unit checks on all computations would help students debug both their programs and their physics understanding.  I really don’t understand why this was not included from the beginning in Vpython, since the language was intended for use as a language for teaching physics. Carrying the units around like this does slow the code down (perhaps by a factor of two), but the pedagogical value of being able to debug incorrect-unit bugs far exceeds the pedagogical value of faster simulations.

Next year, in the circuits course, I’m going to be a stickler about students always including units in their computations, as a consistency check on their thinking, since many of the bioengineers do not have good number sense or algebraic intuition.  Forcing them to carry units throughout may make them a little less likely to do random algebraic manipulations.

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