Motion
Start with one-dimensional change, expand the same kinematic language into two dimensions, then learn how every measurement depends on a chosen observer.
A motion is a history measured from a frame.
Kinematics builds the language of change. Projectile motion shows that the same language works independently along multiple axes. Relative motion reminds us that coordinates and velocities are always measurements from a chosen frame.
Keep the model, change what the model must handle.
Each lesson adds one new complication without discarding the language built before it.
Every coordinate component and every observer describes the same event sequence in time.
Trajectories, component graphs, equations, and vectors should all agree because they encode one shared state.
Coordinates and velocities change with the frame, but the physical event being modeled does not disappear.