What is the difference between speed and velocity?
Short answer
Speed tells you how fast something is moving. Velocity tells you how fast and in which direction. Speed is a scalar quantity (size only) and velocity is a vector quantity (size and direction). A car going round a roundabout at a steady 30 km/h has a constant speed but a changing velocity.
Step by step
- Speed = distance ÷ time, measured in m/s or km/h.
- Velocity = displacement ÷ time, where displacement is the straight-line distance in a given direction.
- Two objects with the same speed have different velocities if they move in different directions.
- A change in velocity, even only in direction, means the object is accelerating.
Worked example
A runner completes one 400 m lap of a track in 80 s, finishing where she started.
- Average speed: 400 ÷ 80 = 5 m/s
- Displacement: 0 m, because she ends at the start
- Average velocity: 0 ÷ 80 = 0 m/s
Common mistakes
- Using the words speed and velocity as if they mean the same thing in an exam answer.
- Forgetting to give a direction when stating a velocity, for example 12 m/s north.
- Thinking constant speed means no acceleration when the direction changes.
Useful for:GCSEKS3BECECheckpoint
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