Describing Motion — Kinematics in One Dimension

Kinematics is the branch of mechanics that describes how objects move without asking why they move. Before we can talk about forces, we need a precise vocabulary for motion itself. That vocabulary rests on four connected quantities: displacement, velocity, acceleration, and time.

Distance versus displacement

Distance is the total length of the path an object travels. It is a scalar, so it carries only a magnitude. Displacement is the straight-line change in position from start to finish, together with a direction. It is a vector. If you jog 400 m around a running track and return to your starting line, your distance traveled is 400 m but your displacement is zero, because your final and initial positions coincide. Keeping these two ideas separate is the single most common source of early e