In everyday conversation, “speed” and “velocity” mean pretty much the same thing. In physics, they are different, and the difference between speed and velocity comes down to one thing: direction. Speed tells you how fast something is moving. Velocity tells you how fast, and which way.
| Speed | Velocity | |
|---|---|---|
| What it tells you | How fast | How fast, and in which direction |
| Type of quantity | Scalar (size only) | Vector (size and direction) |
| Example | 80 km/h | 80 km/h heading west |
| Can it be negative? | No | Yes, if it points the opposite way to the direction you call positive |
| Average over a trip | Total distance ÷ time | Change in position ÷ time |
Scalars and vectors
Physicists split quantities into two kinds. A scalar has a size only: temperature, mass or speed. A vector has a size and a direction: a push to the left, a wind from the south, or a velocity. Speed is simply the size of the velocity, as Georgia Tech’s physics guide puts it, which is why they share the same units, such as kilometres per hour or metres per second. So “the car is doing 80 km/h” is a speed, and “the car is doing 80 km/h heading north” is a velocity.
The running track example
The clearest way to see the difference is on a running track. Imagine you run one lap of a 400-metre athletics track in 80 seconds and finish exactly where you started.
- Your average speed is the total distance divided by the time: 400 m ÷ 80 s = 5 metres per second.
- Your average velocity is your change in position divided by the time. But you finished where you started, so your change in position is zero, and your average velocity is 0 metres per second.
You clearly were not standing still, and your velocity at any single moment was not zero; you were moving at around 5 m/s the whole way. But averaged over the lap, all that movement in different directions cancels out. That is the key idea: average speed cares about how far you travelled, while average velocity only cares about where you ended up compared with where you started. Physicists call that straight-line change in position the displacement.
A road trip example
Now try a drive. You head 30 km north, turn and drive 40 km east, and the whole trip takes one hour.
- Distance travelled: 30 + 40 = 70 km, so your average speed is 70 km/h.
- Displacement: you finish 50 km from where you started, in a straight line roughly north-east (a 3-4-5 triangle, for anyone who remembers their Pythagoras). So your average velocity is 50 km/h, heading north-east.
So the same trip gives you two different answers, and both are correct. Your average speed reflects all the kilometres on the odometer, while your average velocity reflects the shortcut you would have taken if you could have driven in a straight line.
Why it matters: acceleration
The difference also explains something that surprises a lot of people. Drive around a roundabout at a steady 40 km/h and your speed never changes, but your direction does, so your velocity is changing all the time. In physics, any change in velocity is acceleration. So a car going around a corner at constant speed is still accelerating, and that is why you feel yourself pressed towards the outside of the turn. It is also why your car’s speedometer shows speed, not velocity: it measures how fast the wheels turn, and has no idea which way you are facing. At any single instant, your speed is simply the size of your velocity; it is only when you average over a whole trip, or when direction changes, that the two part ways.
The short version
Speed is how fast. Velocity is how fast and which way. Most of the time, in daily life, you can use them interchangeably without anyone minding. But if you are studying physics, or trying to work out why your average velocity for a lap of the oval is zero, the difference matters. It is the same kind of distinction as mass and weight, another pair of words we use loosely every day that mean different things to a physicist. And if you want to see how direction and movement play out on a cosmic scale, my post on the barycentre explains why the Moon does not simply orbit the centre of the Earth.
Sources
- Georgia Tech Physics Book: Speed vs velocity
- Universe Today: What is the difference between speed and velocity?