What stays the same
The pendulum, and the small lie that makes it simple
A pendulum's period does not depend on how far it swings. This is one of the most useful false statements in physics, and it is worth knowing exactly how false.
MechanicsThe angle that throws furthest, and why nobody notices
Forty-five degrees is the answer, and the maximum is so flat that a throw ten degrees off loses almost nothing. Both halves of that are worth drawing.
MechanicsCollisions are easier than forces, and momentum is the reason
Nobody knows what happens inside a collision. Momentum conservation makes that ignorance irrelevant, which is the whole trick — and energy, deliberately, is not conserved.
WavesA wave is a shape that travels, and nothing else does
In a wave on water, no water goes anywhere. What moves is the shape — and separating the two motions is the whole of wave physics.
WavesOnly some notes fit, and that is where discreteness comes from
A string clamped at both ends can vibrate at some frequencies and not others. A continuous object producing a whole-number list is the oldest quantisation in physics.
ElectromagnetismCounting what comes out, and never looking inside
Draw any closed surface. The field crossing it depends only on the charge enclosed — not on where that charge sits, not on its shape, not on anything outside.
RelativityTwo axes, one speed, and a diagram that does the arguing
Put position across and time up, insist that light travels at forty-five degrees for everyone, and nearly every result in special relativity becomes something to read off rather than derive.
RelativityThe clock that has to slow, and why no clock can refuse
One constant speed and one right-angled triangle force a moving clock to tick slower. The argument is Pythagoras, which is what makes it inescapable rather than merely surprising.
RelativityNow is a choice of slicing
Two events happening at the same time is not a fact about the events. It is a fact about who is asking, and different observers slice spacetime at different angles.