Step 01Course 1 · Foundations
Physics is a conversation between models and measurements.
A physical quantity is a number plus a unit. Units are not labels pasted on later: they constrain which equations can possibly be right. If two sides of an equation have different dimensions, the model has failed before any experiment begins.
We will use SI units, estimate orders of magnitude, separate vectors from scalars, and state idealizations openly. Every model has a domain: a projectile equation without drag is useful, but it is not a baseball forecast in a gale.
A reliable workflow is: define the system, choose coordinates, draw interactions, write conservation laws, solve, then check units and limiting cases.