Short answer
Speed measures how fast an object moves, such as 60 miles per hour. Acceleration measures how quickly its velocity changes. A car can keep a steady speed while turning and still accelerate because its direction changes. 1 2
Speed says how fast something moves; acceleration says how its velocity is changing.
On this page
At a glance
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| Question | Speed | Acceleration |
|---|---|---|
| Quantity | Rate of movement | Rate of velocity change |
| SI unit | Meters per second | Meters per second squared |
| Can be zero while moving? | No for a moving object | Yes at constant velocity |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Speed. The magnitude of how rapidly position changes with time, measured as distance per unit time. 1
Acceleration. The rate at which velocity changes with time, including changes in speed or direction. 2
Key differences
Speed is the magnitude of velocity; acceleration is a change in velocity per unit time. Pressing a gas pedal can create acceleration, but so can braking or turning. The word “accelerate” in ordinary speech often means only speeding up, while physics uses the broader definition. 1 2
How to tell them apart
Ask what the number describes. A speedometer shows a speed. A change from 20 to 30 mph over several seconds, or a turn at unchanged speed, indicates acceleration. 1 2
Where they overlap
Both describe motion over time and can be computed from observations of position. A faster object can have little acceleration; a slower one can accelerate strongly. 1 2
Edge cases
An object moving around a circle at constant speed has acceleration toward the center. Negative acceleration depends on the chosen direction and does not always mean “slowing down.” 1 2
Why the distinction exists
Physics needs separate quantities for the state of motion and how that state changes under forces. 1 2
Common misconceptions
High speed does not imply high acceleration. Cruising steadily on a highway can mean near-zero acceleration. 1 2
Examples
A train moving steadily at 50 mph has speed but little acceleration; it accelerates while leaving the station. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Time, Velocity, and Speed. Average velocity.
- OpenStax — Acceleration. Average acceleration.