What is speed?
Speed tells us how much distance is covered in each unit of time. If a car travels a long way in a short time, its speed is high. Common units are metres per second (m/s) and kilometres per hour (km/h).
Key idea
Speed = distance ÷ time. From this we also get: distance = speed × time, and time = distance ÷ speed.
Average speed
On a real journey the speed keeps changing. The average speed is the total distance divided by the total time for the whole journey, not the average of the separate speeds.
Worked example
A car travels 120 km in 2 hours. Find its average speed.
Average speed = 120 ÷ 2 = 60 km/h.
If the car keeps going at 60 km/h for another 1.5 h, distance = 60 × 1.5 = 90 km.
Changing units
To change km/h to m/s, multiply by 1000 (km → m) and divide by 3600 (h → s). To change m/s to km/h, do the reverse.
| 72 km/h | = 72 × 1000 ÷ 3600 | = 20 m/s |
| 5 m/s | = 5 × 3600 ÷ 1000 | = 18 km/h |
On a distance-time graph, a sloping line shows movement and a horizontal (flat) line shows the object is at rest. A steeper line means a faster speed, because more distance is covered in the same time.
When you solve a speed problem, first write down what you know, choose the right form of the formula, and check that time is in the same unit throughout — mixing hours and seconds is a common mistake.
Remember
- Speed = distance ÷ time; keep units matched.
- Average speed uses total distance ÷ total time.
- A flat line on a distance-time graph means stationary.