What is velocity?
Speed is calculated from distance, and distance takes no account of the direction travelled. So speed is a scalar quantity.
That leaves room for a quantity that does account for direction. It is velocity.
Velocity
Velocity is defined as the rate of change of displacement.
velocity = displacement ÷ time
Displacement is a vector, so velocity is a vector too — it has a direction as well as a magnitude.
Speed and velocity compared
| Speed | Velocity | |
|---|---|---|
| Calculated from | distance ÷ time | displacement ÷ time |
| Type | Scalar | Vector |
| Unit | m s−1 | m s−1 |
| Can be negative? | No | Yes — if the direction reverses |
Same unit, different quantity
Speed and velocity are both measured in m s−1, but they are not the same thing. A runner who goes once round a circular track and finishes where they started has a large average speed but an average velocity of zero, because the displacement is zero.