Sciences

Speed and Velocity: The Round Trip That Separates Them

Drive 60 km out and 60 km back in two hours: average speed 60 km/h, average velocity zero. The round trip that makes the scalar-vector split obvious.

Réponse courte

What is the difference between speed and velocity?

Speed is how fast; velocity is how fast and in which direction. Drive 60 km out and 60 km back in two hours and your average speed is 60 km/h while your average velocity is exactly zero.

La réponse en bref
Speed is a scalar: a size only. Velocity is a vector: a size and a direction. The difference shows whenever direction changes. On a 120 km round trip taking two hours the average speed is 60 km/h, but the average velocity is zero, because the displacement is zero.

Ce qui change la réponse

Whether the motion is in a straight line in one direction only — then the size of the velocity equals the speed and the distinction never bites.
Whether you mean average or instantaneous: a speedometer shows instantaneous speed, and the instantaneous velocity is that reading together with the way the car is pointing.
Whether the question says distance or displacement, because that single word decides which of the two is being asked for.

The journey where the two numbers disagree completely

Drive 60 km out of the city, spend the afternoon there, and drive the same 60 km home. Ignore the stop and say the driving took two hours in total. Total distance travelled is 120 km, so the average speed is 120 ÷ 2 = 60 km/h. Nobody argues with that.

Now the other one. Displacement is how far you ended up from where you started, together with a direction — and you ended up on your own driveway. Displacement is 0 km. Average velocity is 0 ÷ 2 = 0 km/h. The car moved for two hours at a perfectly ordinary speed and its average velocity is zero.

A runner on a 400 m track gives the same result in under a minute. One lap in 50 seconds is an average speed of 8 m/s and an average velocity of zero, because the finish line is the start line. Neither number is wrong. They are answers to different questions.

The real difference is underneath: distance and displacement

Speed is distance ÷ time. Velocity is displacement ÷ time. Everything else follows from the difference between those two words. Distance is the whole length of the path travelled and cannot decrease. Displacement is the straight line from start to finish with a direction attached, and it can decrease, and it can be zero, and it can be negative.

Walk 3 km north and then 4 km east. Distance walked is 7 km. Displacement is the hypotenuse of a 3–4–5 triangle: 5 km, on a bearing of 053°. Notice that the displacement is smaller than the distance and carries a direction the distance does not have. It is never larger, and the two are equal only when the path is a straight line that never turns back.

Both are measured in metres and both velocity and speed are measured in metres per second, which is exactly why students conflate them. The units cannot tell you which quantity you are holding.

Why physics bothers with the distinction at all

If the difference were only a matter of bookkeeping on journeys, it would not be worth a lesson. It matters because velocity can change while speed stays fixed, and that has physical consequences.

A car going round a roundabout at a steady 30 km/h has a constant speed and a continuously changing velocity, because its direction is changing at every instant. Changing velocity is acceleration. Acceleration requires a resultant force. So a car moving at constant speed around a bend is being pushed, and the push comes from friction between the tyres and the road — which is why a bend taken too fast in the wet ends badly.

That chain of reasoning is impossible to state without vectors. The Moon orbiting the Earth, a ball on a string, an electron in a magnetic field: all constant speed, all accelerating, all requiring a force. Any student who thinks acceleration means speeding up will get every one of these wrong.

Telling which one an exam question wants

The vocabulary is the giveaway, and it is used precisely. Distance, speed, mass, time, energy and temperature are scalars and never carry a direction. Displacement, velocity, acceleration, force, momentum and weight are vectors and always do.

A negative sign is the other clue. A velocity of −5 m/s is perfectly sensible and means 5 m/s in the direction chosen as negative. A speed of −5 m/s is meaningless. If a question or a graph has gone below the axis, you are working with velocity.

And on a velocity–time graph, the area under the line gives displacement, so an area below the axis counts as negative and can cancel an area above it. That is the round trip again, drawn instead of described — and it is the calculation that separates a candidate who has understood the topic from one who has memorised two formulae.

Questions fréquentes

Can something have constant speed and changing velocity?

Yes, and it is the case worth remembering. Anything moving in a circle at a steady rate does exactly this: the size of the velocity is fixed while its direction changes continuously. It is therefore accelerating, and a resultant force must be acting on it towards the centre of the circle.

Is velocity just speed with a direction written after it?

For instantaneous values, essentially yes — 20 m/s due east. For averages it is not, because average velocity uses displacement rather than distance travelled. That is why an average speed of 60 km/h can sit alongside an average velocity of zero on the same journey.

Do speed and velocity have different units?

No. Both are metres per second in SI units, or kilometres per hour, or miles per hour. This is a common source of confusion, because in most topics a change of quantity brings a change of unit. Here you have to read the word, not the unit.

How is average velocity worked out over a curved journey?

Find the straight-line displacement from the start point to the end point, including its direction, and divide by the total time taken. The shape of the route between them makes no difference at all — only the two endpoints and the clock matter.

Dernière mise à jour 21 août 2026