Threads
Themes
Fields say what an essay is about. Themes say what it rhymes with — the motifs that keep turning up in places that have no business being related.
What stays the same
Conservation laws, and the habit of solving a problem by refusing to look at the middle of it.
The same equation again
A pendulum, a circuit, a molecule and a bridge, all obeying one differential equation and not knowing about each other.
Approximations that lie
The small-angle assumption, the frictionless plane, the point mass — where each is fine, and precisely where it stops being.
Fields, not forces
Replacing action at a distance with something that fills space, and what that buys.
The arrow of time
Nearly every law works equally well backwards. Almost nothing else does, and the gap between those facts is thermodynamics.
Where the model stops
Every picture in physics has a domain of validity, and the interesting physics usually lives at its edge.
Order out of the random
Enormous numbers of unpredictable particles producing quantities you can print on a dial.