Definition · Plain-language
Acceleration
Acceleration is the rate at which the velocity of an object changes over time, including any change in speed or in direction.
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A change in velocity, not just speed
Acceleration measures how quickly velocity changes. Since velocity is a vector, with both size and direction, there are three ways to accelerate: by speeding up, by slowing down, or by changing direction. This is why a car rounding a bend at a steady speed is still accelerating — its direction, and therefore its velocity, is changing. Slowing down is sometimes called deceleration, but in physics it is simply acceleration in the direction opposite to the motion.
How it is measured
Acceleration is defined as the change in velocity divided by the time taken, so its SI unit is metres per second per second, written m/s² or m·s⁻². An acceleration of 3 m/s² means the velocity increases by 3 metres per second every second. Because it is a vector, acceleration has a direction as well as a size; an object slowing down has an acceleration pointing backwards relative to its motion. The acceleration due to Earth’s gravity near the surface is about 9.8 m/s².
The link with force
Acceleration and force are tightly connected: a net force is exactly what causes a mass to accelerate, and the two are linked by the relationship that the force equals mass times acceleration. The bigger the force on a given object, the greater its acceleration; the heavier the object, the smaller the acceleration a given force produces. This is why a powerful engine accelerates a light car briskly but the same engine in a loaded lorry produces far less acceleration.
Key facts
At a glance
- Definition: the rate of change of velocity with time
- Quantity type: a vector — has magnitude and direction
- SI unit: metres per second squared (m/s²)
- Three causes: speeding up, slowing down, or changing direction
- Gravity: near Earth’s surface, about 9.8 m/s² downward
- Linked to force: net force equals mass times acceleration
Common misconceptions
What people often get wrong
Often heard: Acceleration only means going faster.
Actually: Acceleration is any change in velocity, so slowing down and changing direction both count. A braking car and a car turning a corner are both accelerating.
Often heard: An object moving at high speed must be accelerating.
Actually: Speed and acceleration are different. An object moving fast but at constant velocity has zero acceleration; acceleration depends on how velocity changes, not on how large it is.
Often heard: If the net force is zero, an object must be standing still.
Actually: Zero net force means zero acceleration, not zero motion. An object can move at a constant velocity indefinitely with no net force acting, as described by the law of inertia.
Going deeper
