The idea
Choose a positive direction before adding displacements. Distance and speed are scalars: they have magnitude only. Displacement and velocity are vectors: direction matters. An object can travel a long distance and still have zero displacement if it returns to its starting point.
Worked example
A runner travels 60 m east, then 20 m west, in 10 s. Find average speed and average velocity.
- Distance = 60 + 20 = 80 m.
- Taking east as positive, displacement = 60 − 20 = +40 m.
- Average speed = 80 ÷ 10 = 8 m/s. Average velocity = 40 ÷ 10 = 4 m/s east.
A mistake to avoid
Do not use displacement to calculate average speed. Returning to the start gives zero average velocity, but not zero average speed.
Check your understanding
A cyclist completes a 400 m lap in 50 s. What are the average speed and average velocity?
Show the explanation
Average speed = 400 ÷ 50 = 8 m/s. Average velocity is zero because the final and initial positions are the same.
Explore the lesson
This guide connects to How Far and How Fast? in the IB Physics course. The first two published lessons per subject are free previews; further interactive lessons require an account.