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What is acceleration?

Acceleration is the rate of change of velocity with time, measured in metres per second squared.

Acceleration explained

Because velocity is a vector, an object accelerates when its speed changes, its direction changes, or both — circular motion at constant speed is still accelerating.
On a velocity-time graph, acceleration is the gradient and displacement is the area beneath the line.

Key formula

a=v−uta = \dfrac{v - u}{t}

Worked example

A cyclist going from 4 m s−14\,\text{m s}^{-1} to 10 m s−110\,\text{m s}^{-1} in 3 s3\,\text{s} accelerates at 2 m s−22\,\text{m s}^{-2}.

Examiner tip

Negative acceleration means acceleration opposite to the positive direction — call it deceleration only when the object is genuinely slowing.

Acceleration: common questions

What is acceleration due to gravity?
Close to the Earth's surface it is about 9.81 m s⁻², usually rounded to 9.8 or 10 in GCSE questions.
Can something accelerate at constant speed?
Yes — an object moving in a circle at constant speed accelerates because its direction, and so its velocity, keeps changing.

Related terms

Newton's second law

Newton's second law states that the resultant force on an object equals its mass multiplied by its acceleration.

Momentum

Momentum is the product of an object's mass and velocity, and it is conserved in any collision or explosion with no external force.

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