Introduction

Physics is, at its core, an experimental science, and it rests on two pillars: experiment and mathematics. Careful measurement has always pushed the subject forward, often across enormous ranges of scale -- more than two thousand years ago the Greek scholar Eratosthenes worked out the radius of the Earth using little more than shadows and geometry, while the size of a single atom was not actually measured until the early twentieth century. Somewhere between those two extremes sits motion, arguably the most central idea in the whole of physics: it shows up at every level, from the microscopic jostling of particles inside an atom, to ordinary macroscopic objects, all the way up to planets and galaxies. In a very real sense, the entire universe can be described in terms of the many kinds of motion happening within it.

But describing motion precisely takes more than everyday words. If ten athletes run a race, simply calling their performances 'fastest', 'faster', 'average' or 'slowest' is too vague to actually compare them -- their motion has to be quantified, i.e. turned into numbers, before it can be meaningfully analysed. That is exactly the role mathematics plays here: it supplies the precise language in which the magnitude and direction of motion can be expressed and compared, which is exactly the toolkit this unit sets out to build.

Kinematics is the branch of mechanics that describes how things move — position, velocity, acceleration, trajectories — without asking why, i.e. without bringing force into the picture at all (that is the job of dynamics, taken up in the next unit). The word itself comes from the Greek kinema, meaning motion.