[brittohalloran]

Doppler shift. One of the simplest and most deeply useful physics principles. Things you learned in high school that are actually useful way down the road. Basic trigonometry (Sohcahtoa) falls in the same category for me.

[mturmon]

So true. I have been watching an excellent lecture series by Prof. Charles Bailyn (Yale U.) where he goes into some current work in astrophysics (2007) using calculus-free physics. (It's at http://oyc.yale.edu/astronomy/astr-160, and other places like iTunes U.)

It's amazing how deeply useful Doppler shift is; he returns to it thematically at least twice, talking about exoplanets as well as black holes. "We can't see them directly, but we can see them through the way they interact with other things."