Speed — AQA GCSE Physics
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Speed explained
Real journeys are not steady. A walker slows at a kerb and speeds up again, and a car accelerates away from lights, brakes for a junction and waits in traffic, so a graph of its motion is a curve rather than a straight line.
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This is why average speed is the quantity normally quoted: it summarises a varying journey in one number, found by dividing the whole distance by the whole time, and it says nothing about the speed at any particular moment. A car averaging 12 m/s across a town journey may have touched 20 m/s and also stood completely still. When a question asks you to calculate the average speed it is inviting the totals; when it says at this instant it wants a tangent or a single reading.
Your focus
- Calculate an average speed for a journey that includes stationary periods.
- Explain why the average speed of a journey differs from the speed at a given moment.
- Sketch a distance-time graph for a journey in which the speed changes.
Speed exam tips
Marking Points
- one mark for recognising that the speed varies through a journey
- one mark for average speed as total distance divided by total time
- one mark for stating that the average does not give the speed at any particular moment
- one mark for linking a changing speed to a curved distance-time graph
Examiner Tips
- 💡Include time spent stopped in the total time when averaging; it is the usual trap.
- 💡Read the command carefully: average speed needs totals, instantaneous speed needs a tangent.
Common Mistakes
- assuming the speed at the start of a journey applies throughout
- calculating an average from the highest and lowest speeds rather than from totals
- leaving stationary time out of the total time
- describing the distance-time graph of real motion as a straight line