The traffic in the photograph is moving along a busy motorway.
The drivers have to be careful to avoid collisions.
Some vehicles move faster than others. They have different speeds.
Drivers on a motorway must obey the speed limit. There are signs along the motorway which show the speed limit.
The sign in the photograph shows that the speed limit is $130 kilometres$ per hour in dry weather.

What is speed?

Speed is the quantity that tells us how fast something is moving For example, we might say:
"The car's speed was $50 kilometres$ per hour when it hit the wall.'
Remember that, when we give a measurement, we give its value including the unit. In this example, the unit is kilometres per hour, usually written as $km/h$.
A speed of $50km/h$ means that the car would travel $50km$ in one hour if it kept moving at that speed.
In science, we usually measure speed in metres per second ($m/s$).
So we might say:
"The sprinter's speed was $10m/s$.'

Distance and time

How can we measure a runner's speed? There is a clue in the units. We need to find how many metres a runner travels in each second. So we need to measure two quantities:
• distance travelled (in metres, m)
• time taken in seconds, s).

Then we calculate speed like this:

$\begin{array}{l}\begin{array}{*{20}{c}}{average}&{speed = \frac{{\begin{array}{*{20}{c}}{distance}&{travelled}\end{array}}}{{\begin{array}{*{20}{c}}{time}&{taken}\end{array}}}}\end{array}\\\begin{array}{*{20}{c}}{Or}&{simply}\end{array}\\speed = \frac{{distance}}{{time}}\end{array}$

We have to say average speed because the runner's speed may be changing as they run - they may be speeding up or slowing down.

Calculating speed

Here is an example. A runner completes a $200m$ race in $25s$. What is her average speed?

$speed = \frac{{distance}}{{time}} = \frac{{200m}}{{25s}} = 8m/s$