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Speed vs velocity
Beginner
Speed is a scalar; velocity includes direction.
v = Δx/ΔtSpeed says how fast; velocity says how fast and which way. A car going round a circle at constant speed has changing velocity because its direction keeps changing.
A car that does 60 km/h north then 60 km/h south has the same speed but velocities of opposite sign.
Velocity can be negative (direction), speed cannot.
In more detail
Speed is the magnitude of velocity and has no direction. Velocity includes direction, so a car turning a corner at constant speed still has changing velocity.
Real-world uses
- A speed camera measures speed, but it does not tell you the direction a car is heading.
- Pilots track velocity so they know both how fast and which way they are flying.
- A speedometer in a car shows speed, but a satnav needs velocity to tell you whether you are heading toward your destination.
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Frequently asked questions
Can speed be negative?
No. Speed is always zero or positive. Velocity can be negative when the motion is in the negative direction.
Can an object have constant speed and changing velocity?
Yes, when it moves in a circle at a steady speed, its direction keeps changing.
If speed is constant, is velocity constant?
Not if the direction changes. Driving round a roundabout at steady speed means the velocity keeps changing.
Related topics in Kinematics
Rest and motion · Reference frames · Distance vs displacement · Average velocity · Instantaneous velocity · Acceleration