Kinematics

In the last chapter you taught a dot to move along a line — one number, one rule. Now let's give it somewhere to go.

Picture yourself floating in a completely empty, pitch-black universe. No stars, no ground, no up. You toss a pebble. Where is it? "Over there" means nothing here — there's no there yet.

So we build one. Two numbers this time: x across and y up-and-down. Together they pin down any point on the plane.

The Lander on the right has a position. Press Play and it holds it, patiently. But a place to be was never the same thing as a reason to leave. For that we need change again — and this time it has a name.