Physics: finding the rules the world obeys
Drop a stone and it falls; push a cart and it rolls; heat a pot and it boils. Physics is the ambitious bet that behind all of this lies a small set of exact, universal rules — rules you can write down as equations and put to the test against nature.
The key word is quantitative. Physics is not content to say things fall; it asks how fast, and demands a formula that predicts the answer every single time. The same handful of laws governs a speck of dust and a spiral galaxy — that reach is exactly what makes the subject worth learning.
How physics is actually done: the scientific method
Knowledge in physics grows through a cycle — the scientific method: notice a pattern, propose a hypothesis or model, work out a testable prediction, measure to check it, then keep, revise, or discard the idea. Experiment is the final judge; no equation is believed because it is elegant, only because it agrees with what is measured.
Physicists win by simplifying on purpose: a planet becomes a point mass, a surface becomes frictionless, a real gas becomes an ideal gas. A good model keeps what matters and throws away the rest — but you must always know what you threw away. This honesty about idealizations runs through the entire subject.
Every measurement is a number and a unit
The atom of physics is the physical quantity — anything you can measure, such as length, time, mass, temperature, or electric current. Its value is always a number multiplied by a unit. The bare number 3 tells you nothing until you say 3 metres, or 3 seconds, or 3 kilograms.
Value and unit are inseparable: change the unit and the number must change to match.
For example, a car that covers 100 m in 4 s has a speed v = d/t = 25 m/s — and both the 25 and the 'm/s' are essential. Some quantities carry direction as well as size: a scalar has magnitude only (mass, time, temperature), while a vector has magnitude and direction (displacement, velocity, force). We build that vector language in Guide 5.
Why getting units right is not optional
An honest cautionary tale: in 1999 the Mars Climate Orbiter was lost because one team supplied a thruster's impulse in pound-force-seconds while the navigation software expected newton-seconds. The mismatch nudged the spacecraft too deep into the Martian atmosphere, and it was destroyed. It was a units bug, not a physics bug.
This track builds the toolkit before any real physics. Guide 2: the SI system of units. Guide 3: scientific notation, orders of magnitude, and dimensional analysis for reasoning about size. Guide 4: significant figures and uncertainty — how to report a measurement honestly. Guide 5: the scalar/vector language that kinematics, dynamics, and fields are all written in.