MotionOxford AQA IGCSE Physics: Revision notes
Section 1
How do we show motion on a distance-time graph?
A distance-time graph plots the distance of an object from a fixed point against time.
- The gradient (slope) of the line represents the speed of the object
- A flat (horizontal) section means the object is stationary
- A straight, sloped line means the object is moving at constant speed
- A curved line means the speed is changing (non-uniform motion)
- A steeper gradient means a faster speed
Examiners want you to state that gradient = speed explicitly, not just describe the graph shape.
Section 2
How do you calculate speed from a distance-time graph?
Speed is calculated from the gradient of a distance-time graph:
speed = change in distance ÷ change in time
To find the gradient at any point:
- Choose two points on the line
- Find the change in distance (vertical change) between them
- Find the change in time (horizontal change) between them
- Divide the change in distance by the change in time
If an object travels 40 m in 8 s on a straight-line section of a distance-time graph, its speed = 40 ÷ 8 = 5 m/s.
Section 3
What is velocity and how does it differ from speed?
Velocity is speed in a given direction. This makes velocity a vector quantity, while speed is a scalar quantity (magnitude only).
Velocity is calculated using:
v = s ÷ t
where v is velocity, s is displacement, and t is time.
Displacement is the straight-line distance from the start point to the finish point, in a specified direction — this is different from total distance travelled, which does not need a direction.
Students often use 'speed' and 'velocity' interchangeably — in an exam answer, only use 'velocity' when direction is relevant or specified.
Section 4
How do you find average speed for non-uniform motion?
Real journeys rarely happen at constant speed. For non-uniform motion (changing speed), the equation v = s ÷ t is used to find the average speed over the whole journey, not the speed at any single instant.
average speed = total distance travelled ÷ total time taken
This is different from instantaneous speed, which is the speed at one particular moment (found from the gradient of a distance-time graph at that exact point).
When a question gives total distance and total time for a journey with varying speed, use v = s/t to find the average speed, not the speed at any one point.
Must Know
- Gradient of a distance-time graph = speed
- A flat line on a distance-time graph means the object is stationary
- A curved line means the speed is changing
- Velocity = speed in a given direction; velocity is a vector, speed is a scalar
- v = s ÷ t calculates velocity (or average speed for non-uniform motion)
- Average speed = total distance ÷ total time
That's the notes covered.
Carry on to the next subtopic.