Position
Before physics can say anything about motion, it has to answer a child's question: where is the thing? The answer is always an address measured from a zero that somebody chose – and once you see that the choice is yours, minus signs stop being threats and start being directions.
Intro: “where are you?” answered the human way (“by the lamppost”) and the physics way (“x = +3 m”) – same information, one of them fit for equations.
3.1Zero is a choice
Every address needs a reference point. Physics calls it the origin, marks it x = 0, and – the liberating part – puts it wherever is convenient: the lamppost, the start line, the centre of the Earth. Nothing physical changes when the choice changes, a quiet first taste of the relativity idea that Course III turns into a theory.
3.2The minus sign is a direction
One line through the origin: one side positive, the other negative, by convention and nothing deeper. x = −2 m is not less real than x = +2 m; it is the same distance, the other way. Read a few addresses aloud until the sign feels like “left”.
3.3Distance walked is not where you are
The figure’s lesson: the odometer (every metre, direction blind) and the displacement arrow (net result, sign and all) are both honest and routinely disagree. Walk five right, five left: odometer ten, displacement zero. Physics almost always wants the arrow – the first hint of the vector idea the classical course will lean on.
3.4Position is a snapshot
x tells you where, and nothing else – not where you were, not where you are going. A single frame of the film. The obvious next question (“how does x change?”) is the next chapter, and its answer is the primer’s biggest idea.