2.1.1—Distance, displacement, speed, velocity and acceleration

Distance is total path length, a scalar. Displacement is the directed change from initial to final position, a vector. Speed is distance per time; velocity is displacement per time.

Choose a sign convention for one-dimensional motion and distinguish average quantities from instantaneous derivatives.

A runner completes one 400 m lap and returns to the start: distance is 400 m but displacement is zero.

Zero displacement does not mean the object never moved, and speed cannot be negative.