Velocity

When something moves, we often want to know how fast it moves. The average velocity is the distance travelled divided by the time it took:

v = \frac{\Delta s}{\Delta t}

where $\Delta s$ is the change in position (in metres) and $\Delta t$ is the time interval (in seconds). The SI unit of velocity is therefore $\text{m/s}$ .

Worked example

Worked example: A car on the motorway

A car travels $120\ \text{km}$ in $1.5\ \text{h}$ . What is its average velocity?

Try it yourself

Change the distance and the time and see how the velocity changes.

Distance Δs (m)

Time Δt (s)

Velocity v10.00 m/s

Position–time graph

For constant velocity, position grows linearly with time. The slope of the line is the velocity.

Velocity v (m/s)2

Show values as a table

t (s)	v * t
0	0
1	2
2	4
3	6
4	8
5	10
6	12
7	14
8	16
9	18
10	20

Exercises

Exercise

Which is the SI unit of velocity?

Exercise

A car travels $120\ \text{km}$ in $1.5\ \text{h}$ . What is its average velocity? Give the answer with a unit.

ExerciseDifficulty 2/5

A cyclist rides $80\ \text{m}$ in $27\ \text{s}$ . What is the average velocity in m/s?