Movement & PositionEdexcel IGCSE Physics: Revision notes
Section 1
Distance-time graphs
A distance–time graph plots the distance travelled by an object against time. Key features:
- A flat (horizontal) line means the object is stationary.
- A straight sloped line means constant speed — the steeper the line, the faster the object is moving.
- A curved line means the speed is changing (accelerating or decelerating).
- The gradient of a distance–time graph gives the speed of the object.
average speed = distance moved / time taken
When asked to describe a distance-time graph, refer to specific sections (e.g. "between 0 and 5 s") and state whether the object is stationary, moving at constant speed, or accelerating.
Section 2
Practical: investigating motion
The motion of everyday objects, such as toy cars or tennis balls, can be investigated by measuring the distance travelled at regular time intervals (e.g. using a metre ruler and stopwatch, or light gates) and plotting the results on a distance–time or velocity–time graph.
Repeating measurements and taking an average improves reliability. Using light gates instead of a stopwatch reduces human reaction-time error.
Reaction time when starting/stopping a stopwatch is a major source of error in these experiments — always mention it if asked to evaluate the method.
Section 3
Velocity-time graphs and acceleration
Acceleration is the rate of change of velocity:
a = (v − u) / t
where v is final velocity, u is initial velocity and t is time taken. Acceleration is measured in m/s².
On a velocity–time graph:
- The gradient gives the acceleration.
- A flat line means constant velocity (zero acceleration).
- A line sloping upward means the object is speeding up; sloping downward means it is slowing down (decelerating).
- The area between the line and the time axis gives the distance travelled.
A car accelerates from 0 to 20 m/s in 5 s. a = (20 − 0)/5 = 4 m/s².
For a velocity-time graph with a triangular or trapezoidal shape, split the area into simple shapes (triangles and rectangles) and add them together to find total distance.
Section 4
The equation of motion v² = u² + 2as
When time is not known, the relationship
v² = u² + (2 × a × s)
can be used to relate final speed (v), initial speed (u), acceleration (a) and distance moved (s). This is especially useful in stopping-distance and braking calculations.
A car travelling at u = 10 m/s accelerates at 2 m/s² over a distance of 24 m. v² = 10² + (2 × 2 × 24) = 100 + 96 = 196, so v = 14 m/s.
Must Know
- average speed = distance moved / time taken
- gradient of a distance-time graph = speed; flat line = stationary; curve = changing speed
- acceleration, a = (v − u)/t, measured in m/s²
- gradient of a velocity-time graph = acceleration; area under it = distance travelled
- v² = u² + (2 × a × s) is used when time is not given
- practical investigations of motion should use repeated readings and consider reaction-time error
That's the notes covered.
Carry on to the next subtopic.