Interactive physics lesson
Speed vs Velocity
Speed tells how fast an object moves. Velocity tells how fast it moves and in which direction. Average speed uses total distance, while average velocity uses displacement.
Start here
What is the difference between speed and velocity?
In everyday conversation, people often use speed and velocity as if they mean the same thing. In physics, they answer different questions.
Easy memory trick: velocity is speed with a direction.
| Feature | Speed | Velocity |
|---|---|---|
| Meaning | How fast an object moves | How fast an object moves and its direction |
| Type of quantity | Scalar: magnitude only | Vector: magnitude and direction |
| Average formula | Total distance / total time | Displacement / total time |
| Direction needed? | No | Yes |
| Can it be negative? | No | A component can be negative relative to a chosen positive direction |
| SI unit | metre per second, m/s | metre per second, m/s, plus direction |
| Example | 12 m/s | 12 m/s west |
The foundation
Distance and displacement decide which formula to use
Many speed and velocity mistakes begin by mixing up distance and displacement. Learn this pair first.
Distance
Distance is the full length of the path travelled. It does not include direction and cannot become smaller during one recorded journey.
Displacement
Displacement is the change from starting position to final position, measured in a straight line and with direction.
Calculate it
Speed and velocity formulas
Average speed formula
Average speed = total distance / total timeUse every part of the path. Direction does not enter the calculation.
Average velocity formula
Average velocity = displacement / total timeUse only the net change in position, then state its direction.
Units: Both quantities have the SI unit m/s. Kilometres per hour is also common. To convert km/h to m/s, divide by 3.6. To convert m/s to km/h, multiply by 3.6.
Average is not always the middle of two speeds
Do not automatically add two speeds and divide by two. The reliable rule is total distance divided by total time. A simple arithmetic mean works only in special cases, such as when the object spends equal time at each speed.
Worked examples
Two trips that reveal the difference
A bicycle travels 120 m east in 20 s
- Distance = 120 m
- Displacement = 120 m east
- Average speed = 120 / 20 = 6 m/s
- Average velocity = 120 / 20 = 6 m/s east
Here, the speed equals the magnitude of the velocity because the bicycle travels straight without reversing direction.
A runner goes 100 m east and 100 m west in 40 s
- Distance = 100 + 100 = 200 m
- Displacement = 0 m
- Average speed = 200 / 40 = 5 m/s
- Average velocity = 0 / 40 = 0 m/s
The runner moved throughout the trip, but the average velocity is zero because the final position is the starting position.
Build a journey and watch speed separate from velocity
Choose a distance and time for each leg. Move east or west, then compare total distance with displacement. In this model, east is the positive direction. The return button uses the pace selected by the two sliders.
This is a mathematical simulation for learning. Its readings are calculated values, not measurements from a real experiment.
Start at 0 m. Try moving east and then west.
- Started at 0 m.
Can speed stay constant while velocity changes?
Yes. During uniform circular motion, an object's speed can remain constant while its velocity changes continuously because its direction changes. The red arrow shows the instantaneous velocity, which is tangent to the circular path.
This diagram is a simplified simulation. It demonstrates direction, not a measured orbit.
At the top of the circle, clockwise velocity points east.
Do not mix these
Average and instantaneous values
Instantaneous speed
How fast an object is moving at a particular moment. A car speedometer displays an estimate of instantaneous speed.
Average speed
Total distance divided by total elapsed time for the whole interval, including stops if they are part of that time.
Instantaneous velocity
The object's speed and direction at a particular moment. Its magnitude is the instantaneous speed.
Average velocity
Displacement divided by total elapsed time. It describes net change in position, not every movement made on the way.
Hands-on investigation
Measure a one-way walk and a round trip
Question: How does returning to the starting point affect average speed and average velocity?
Hypothesis: A round trip will have a positive average speed but zero average velocity because its displacement is zero.
Materials
- Measuring tape
- Stopwatch or phone timer
- Two floor markers
- Notebook and pencil
Variables
- Independent variable: one-way or round-trip route
- Dependent variables: calculated average speed and average velocity
- Controls: same walker, marked distance, surface, and timing method
Procedure
- Mark a straight 5 m path and call the forward direction east.
- Walk from the start to the 5 m marker. Record the time.
- Calculate distance, displacement, average speed, and average velocity.
- Reset the timer. Walk 5 m east and then return 5 m west to the start.
- Record the total time and calculate the four quantities again.
- Repeat each route three times if you want a more dependable comparison.
Expected observation
The one-way trip has a 5 m east displacement. The round trip covers 10 m of distance but has 0 m displacement. Therefore, its average speed is above zero while its average velocity is 0 m/s.
| Trial | Route | Distance | Displacement | Time | Average speed | Average velocity |
|---|---|---|---|---|---|---|
| 1 | 5 m east | 5 m | 5 m east | Record | Calculate | Calculate east |
| 2 | 5 m east, then 5 m west | 10 m | 0 m | Record | Calculate | 0 m/s |
Safety: Use a clear, dry, level path. Walk instead of sprinting, keep away from stairs and traffic, and ask an adult or teacher to supervise if needed.
Mistake prevention
Common speed and velocity mistakes
Average velocity uses displacement, not the full route length.
A velocity answer is incomplete without direction or a clearly defined sign.
Speed is a magnitude, so it is never negative. A negative velocity component indicates direction.
Use total distance divided by total time unless equal time intervals make a simple mean valid.
If a stop occurs within the measured trip, its duration belongs in total elapsed time.
Constant speed around a circle still means changing velocity because direction changes.
Student challenge
Practice problems with checked answers
Try each question before opening its answer.
1. A runner covers 90 m in 15 s. What is the average speed?
Average speed = distance / time = 90 / 15 = 6 m/s.
2. A student walks 60 m east and then 20 m west in 40 s. Find average speed and average velocity.
Total distance = 60 + 20 = 80 m. Displacement = 60 m east - 20 m west = 40 m east.
Average speed = 80 / 40 = 2 m/s. Average velocity = 40 / 40 = 1 m/s east.
3. A car completes one 400 m lap in 80 s and returns to the starting line. Find both averages.
Average speed = 400 / 80 = 5 m/s. Displacement is zero, so average velocity = 0 / 80 = 0 m/s.
4. East is positive. A skateboarder changes position by -30 m in 10 s. What is the average velocity?
Average velocity = -30 / 10 = -3 m/s. The negative sign means west, so the answer may also be written as 3 m/s west.
5. A bus travels for equal times at 30 km/h and 50 km/h. What is its average speed?
Because the time intervals are equal, the arithmetic mean is valid: (30 + 50) / 2 = 40 km/h. If the distances were equal instead, this shortcut would not be valid.
Knowledge check
Speed vs velocity quiz
Select one answer for each question, then check your score.
Quick answers
Frequently asked questions
Can speed and velocity have the same value?
The numerical value of speed can equal the magnitude of velocity when motion is along a straight path without reversing direction. Velocity still includes a direction, while speed does not.
Can average velocity be zero while an object is moving?
Yes. If the object returns to its starting position, its displacement for the whole interval is zero. Its average velocity is then zero even though its distance and average speed are positive.
Can an object's instantaneous velocity be zero?
Yes. An object can be momentarily at rest, such as at the instant when it changes direction at a turning point. This is different from having zero average velocity over a longer interval.
Does a speedometer show speed or velocity?
A speedometer shows instantaneous speed because it gives a magnitude without direction. A navigation system may combine speed with a direction of travel to describe velocity.
Why does velocity change in circular motion?
Velocity is a vector. Even when its magnitude, the speed, stays constant, the direction of motion changes at every point on the circle, so the velocity changes.
Is average speed always greater than average velocity?
Average speed is greater than or equal to the magnitude of average velocity. It is equal for straight motion without reversing direction and greater when the path length exceeds the magnitude of displacement.
Final summary
- Speed is a scalar that tells how fast an object moves.
- Velocity is a vector that tells speed and direction.
- Average speed uses total distance; average velocity uses displacement.
- Both use m/s in SI, but a velocity answer also needs direction.
- A round trip can have positive average speed and zero average velocity.
- Constant speed does not guarantee constant velocity when direction changes.
Continue learning
Explore more motion lessons on SpeedUpScience
Fact check
Scientific sources
The definitions, formulas, examples, and SI units on this page were checked against established physics and measurement references.
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