force
Force begins as a push you can feel and a vector you can add — Newton's F = ma. It hardens into the gradient of a potential, then dissolves: in quantum mechanics no force acts, yet expectation values still obey Newton's law; in general relativity gravity is not a force at all but the curvature of spacetime that free bodies simply follow.
Units touched PU-201 · PU-202 Adjacent threads energy · fields · symmetry Stations 2
The stations
Central-force orbit equation
Force is still a vector, but symmetry does the heavy lifting: angular momentum is conserved, the two-body problem collapses to one radial coordinate, and F(r) alone fixes the shape of every orbit.
Ehrenfest's theorem & the classical limit
The particle vanishes into a wavefunction and no force acts on it — yet d⟨p⟩/dt = −⟨∇V⟩. Newton's second law re-emerges as a statement about averages, showing exactly how and when "force" survives the quantum world.