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Acceleration
Beginner
The rate at which velocity changes.
a = dv/dtAcceleration is the rate of change of velocity, a = dv/dt. Since velocity is a vector, a change in direction also counts as acceleration. Speeding up, slowing down and turning are all accelerations.
A car increases from 0 to 27 m/s in 6 s: a = 4.5 m/s².
Negative acceleration doesn't always mean slowing down; it depends on the sign of velocity too.
In more detail
Acceleration is the rate of change of velocity, which can mean speeding up, slowing down or changing direction. It is a vector, so turning at constant speed is still acceleration.
Real-world uses
- Car makers quote 0 to 100 km/h times as a measure of average acceleration.
- Lift designers limit acceleration to keep passengers comfortable.
- A car's acceleration figure, such as 0 to 100 km/h in 6 seconds, describes how quickly its velocity changes.
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Frequently asked questions
Does negative acceleration always mean slowing down?
No. It means acceleration points in the negative direction. If the velocity is also negative, the object is speeding up.
What are the units of acceleration?
Metres per second squared, m/s².
Does a slowing car have negative acceleration?
Only if the positive direction is taken as the direction of motion. Acceleration is the change in velocity, whatever the sign.
Related topics in Kinematics
Speed vs velocity · Average velocity · Instantaneous velocity · Uniform motion · Position–time graphs · Velocity–time graphs