Motion and graphs is the part of Physics that describes how things move, and how to read that movement from a line on a grid. It rests on two ideas: speed and acceleration as rates, and graphs as pictures of those rates.
It is an early topic in Cambridge IGCSE Physics, and later work on forces, momentum and energy keeps returning to it. A student who reads motion graphs confidently finds forces and momentum much easier.
What should I know before starting?
You need comfortable arithmetic with division and units, and the ability to read a value from a graph axis. Units from measurement and quantities matter here, especially seconds, metres and the difference between km/h and m/s.
If you can find the gradient of a straight line as “rise over run”, you already have the main maths skill.
An orienting example
Here is invented data for a school bus (made up for practice, not a real timetable). It travels 300 m in the first 20 s at steady speed, waits at a stop for 10 s, then travels 150 m in the next 10 s.
- First segment: speed = 300 m ÷ 20 s = 15 m/s.
- Stop: distance does not change, so speed = 0 m/s.
- Last segment: speed = 150 m ÷ 10 s = 15 m/s.
- Whole journey: distance = 450 m, time = 40 s, average speed = 450 ÷ 40 = 11.25 m/s.
On a distance-time graph the bus gives a rising line, a flat line, then a rising line again. The steepness is the speed.
The average speed (11.25 m/s) is lower than the moving speed (15 m/s) because the stop is included. That one example already contains ideas from three lessons in this module.
In which order should I study the lessons?
- Calculate speed over a segment. Start here, because every graph rule later is built on speed = distance ÷ time.
- Compare distance-time and speed-time graphs. Learn which graph answers which question before you calculate anything from one.
- Find acceleration from a gradient. This uses the speed-time graph and introduces the unit m/s².
- Interpret area under a speed-time graph. Area gives distance, which links back to the first lesson.
- Distinguish displacement from total distance. This adds direction and explains why the two quantities can differ.
Then attempt the motion and graphs practice set and revisit whichever lesson your mistakes point to.
What are the common traps?
- Reading the wrong axis. A flat line on a distance-time graph means “at rest”, but a flat line on a speed-time graph means “constant speed”.
- Forgetting units. Speed is in m/s, acceleration in m/s². Writing “2 m/s” for an acceleration loses the mark.
- Using the wrong formula on a slope. Dividing a final speed by a time only works if the object started from rest.
- Mixing distance and displacement. Total distance can be larger than the straight-line change in position.
How should I use the practice set?
Attempt each question with the solution closed, write a full answer including units, and only then open the working. Mark yourself honestly on method as well as on the final number.
The graph-model and residual explorer and the rate and energy graph interpreter on this site let you test your reading of a graph. The mistake log and retest queue helps you track which kind of error you keep repeating.
Students who understand each lesson but still misread graphs under time pressure often benefit from explaining their reasoning out loud to a teacher. That is one of the things our teachers do in online one-to-one Physics tuition.