If you watch someone compete in a 100 metre sprint, you could time how long it takes them to run that distance and try and calculate their speed.

But does that sprinter run at a **constant speed **throughout the race? No, the sprinter starts slower, then gets faster and faster over the first few seconds of the race.

Instead, what we could do is calculate the **average speed **of the sprinter, using the total distance travelled, and the total time taken. Let's learn about how we can calculate average speed!

What is speed?

Speed measures how fast something travels - speed tells us how far something travels in a given time. We can calculate speed using this equation:

Speed can be measured in lots of different units, but the **standard unit **is **metres per second** (m/s).

Distance is measured in **metres**.

Time is measured in **seconds**.

When objects move, they sometimes move at **constant speeds. **At a constant speed, an object moves the same distance every second. For example, if you run for a bus at 3 m/s, that means that every second you travel 3 metres.

Often, the speed of an object might not be constant. Objects can speed up, getting faster and faster. That is acceleration. Objects can also slow down - that is called deceleration.

It is useful to be able to calculate the **average speed **of an object, over its whole journey.

We can calculate the **average speed **using the speed equation listed above. We need to take the **overall distance travelled, **and divide it by the **overall time taken. **

Let's look at an example.

__Example__

A train travels a distance of 3,500 metres, in a time of 125 seconds.

What is the average speed of the train?

**Answer**

First, let's make sure we write down the right equation we need to use.

Average speed (m/s) = Total distance (m) ÷ Total time (s)

Average speed (m/s) = 3,500 m ÷ 125 s

Average speed = **28 m/s**

Now that we understand the concept of average speed, let's try some questions!