Movement & Position
Forces & Motion
What speed measures
- Speed is the distance an object travels each second
- Speed is a scalar quantity: it has a magnitude (a numerical size) but no direction
- The SI unit is metres per second (m/s); other units include km/h and mph for everyday contexts
- An object's speed is rarely truly constant from second to second; in most real journeys the instantaneous speed drifts as the motion progresses
Average speed
- smooths the whole journey into one figure that ignores the moment-by-moment variation:
average speed = (total distance moved) / (total time taken)
- The same equation rearranges two ways:
- distance = average speed × time
- time = distance / average speed
Common exam question
Stating and using the average speed equation
Question: State the equation that links average speed, distance moved and time taken, then use it to calculate a speed, a distance or a time (1 mark, then 2–3 marks).
The equation is asked for in 3 of the 24 papers. Write average speed = distance moved ÷ time taken; standard symbols and any correct rearrangement are accepted, and the schemes allow s or d for distance and v or s for speed. The calculation gives one mark for the substitution (or the rearrangement, when the unknown is the distance or the time) and one for the evaluation, so show the substitution.
When a speed limit is in km/h or the data are in miles per hour, converting is a mark in itself: multiply the miles by the metres-per-mile factor given and divide by 3600 to reach m/s, or convert your m/s answer into km/h. Either direction is accepted when the conversion is shown and your conclusion matches your numbers.