Speed and motion
Describe how quickly an object moves using consistent measurements and units.
What you will learn
- Distinguish speed from velocity.
- Calculate average speed from a journey.
Distance and displacement
Distance is the total path length travelled. Displacement describes the change in position with direction. Speed uses distance; velocity uses displacement and direction. An object returning to its starting point can have travelled a large distance while having zero overall displacement.
Use compatible units
Average speed is total distance divided by total time, including any stops if they form part of the journey. Convert kilometres to metres and minutes to seconds before substituting when the answer requires m/s.
Constant speed means equal distances in equal times. On a distance–time graph, the gradient gives speed; a horizontal section represents no change in distance.
Worked example
A runner travels 360 m in 90 s. Find the average speed.
Show the worked solution
- Both measurements already use metres and seconds.
- Divide total distance by total time: 360/90.
- Attach the units m/s.
Answer 4 m/s
Common mistakes
- Do not divide by minutes when an answer in m/s is required.
- The average of two speeds is not generally the average speed of the whole journey.
What can you explain now?
A cyclist travels 1.2 km in 4 minutes. Find average speed in m/s.
Compare with the explanation
5 m/s
Convert to 1200 m and 240 s; 1200/240=5.
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