Definition · Plain-language
Velocity
Velocity is the rate at which an object changes its position in a particular direction — speed together with direction.
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Speed with a direction
Velocity tells you not just how fast an object moves but which way it is going. It is defined as the change in position (displacement) divided by the time taken, which makes it a vector quantity with both a magnitude and a direction. The magnitude of the velocity is the object’s speed, so "10 metres per second east" is a velocity while "10 metres per second" alone is just a speed. This direction is what makes velocity more informative — and more useful in physics — than speed.
Average and instantaneous velocity
Average velocity is the total displacement divided by the total time for a journey, describing the overall change in position. Instantaneous velocity is the velocity at one precise moment, which is what a speedometer needle approximates at each instant (giving the speed) together with the heading. The two can differ greatly: on a winding route the instantaneous velocity changes constantly as direction shifts, while the average velocity simply connects start to finish. Over a round trip back to the start, average velocity is zero even though the object never stopped moving.
Why changing velocity means acceleration
Because velocity includes direction, it changes whenever speed changes, direction changes, or both. Any change in velocity is acceleration, which is why an object travelling at constant speed around a bend is accelerating — its direction, and so its velocity, is constantly turning. This is the key reason physics insists on the distinction between speed and velocity: forces and acceleration act on velocity, the directional quantity, not on bare speed.
Key facts
At a glance
- Definition: rate of change of position in a stated direction
- Quantity type: a vector — speed plus direction
- SI unit: metres per second (m/s)
- Based on: displacement, not total distance travelled
- Versus speed: speed is the magnitude of velocity, without direction
- Changing velocity: any change in speed or direction is acceleration
Common misconceptions
What people often get wrong
Often heard: Velocity and speed are the same thing.
Actually: Speed is how fast an object moves; velocity is how fast and in which direction. Velocity is a vector built on displacement, so two objects with the same speed can have different velocities if they move different ways.
Often heard: Constant speed means constant velocity.
Actually: Only if the direction is also unchanged. An object moving at a steady speed around a curve has a constantly changing velocity, because its direction is changing — and that means it is accelerating.
Often heard: Average velocity equals average speed over any journey.
Actually: They match only for motion in a straight line that never reverses. On a route that changes direction, average velocity (based on displacement) is smaller than average speed (based on total distance), and can even be zero.
Going deeper
