displacement

Displacement is how far and in what direction an object ends up from where it started. Imagine you walk out your front door, wander around the block, and come back to the door. You have walked a long way, yet your displacement is zero, because you finished exactly where you began. Displacement answers: what is the straight-line change from start to finish, including direction?

Precisely, displacement is the change in position: final position minus initial position, written as an arrow (a vector) from the start point to the end point. In one dimension, displacement = x_final - x_initial, often written as delta x. Its size is the straight-line distance between start and end, and its direction points from start toward end. Because it is a subtraction of positions, displacement does not depend on where you put the origin, and its SI unit is the metre (m).

Displacement is the raw material of velocity: average velocity is displacement divided by the time it took. The key honesty here is that displacement is not the same as the total path you walked. Two people can travel wildly different paths yet have identical displacement if they start and end at the same two points. Only when the motion never reverses direction do the displacement's size and the distance travelled agree.