Unit 4: Describing Motion (Kinematics)
This unit is about describing movement using numbers and graphs: how fast something goes, whether it is speeding up, and how to read distance–time and speed–time graphs.
01 Speed and velocity
speed (m/s) = distance (m) ÷ time (s)
Speed only tells you how fast. Velocity tells you how fast and in which direction. A car going 50 km/h north has a different velocity from one going 50 km/h south, even though their speed is the same.
02 Acceleration
acceleration (m/s²) = change in velocity (m/s) ÷ time taken (s)
Acceleration is how quickly velocity changes. It can mean speeding up, slowing down (negative acceleration, or "deceleration"), or changing direction.
03 Distance–time graphs
On a distance–time graph, the gradient (slope) of the line equals the speed.
- Flat, horizontal line → object is stationary (not moving)
- Straight, sloped line → constant speed
- Curved line, getting steeper → speeding up
04 Speed–time graphs
On a speed–time graph, the gradient equals the acceleration, and the area under the graph equals the distance travelled.
Tip: a horizontal line on a speed-time graph means constant speed — zero acceleration — even though the object is still moving!
V Key vocabulary
| Term | Meaning |
|---|---|
| Speed | How fast an object moves, distance divided by time. |
| Velocity | Speed in a stated direction. |
| Acceleration | The rate at which velocity changes. |
| Gradient | The steepness of a line on a graph; found as rise ÷ run. |
? Quick check
Try to answer before you tap. Answers are hidden underneath.
A cyclist travels 300 m in 60 s. What is their average speed?
5 m/s (300 m ÷ 60 s = 5 m/s).
On a distance–time graph, what does a horizontal (flat) line mean?
The object is not moving — it is stationary, because distance is not changing over time.
On a speed–time graph, how do you find the total distance travelled?
Calculate the area under the graph line.