What is acceleration?

The basic idea

In physics, acceleration measures the rate at which an object’s velocity changes over time. Note that a change in speed does not simply mean ‘going faster’: it can also mean slowing down, or even changing direction without changing pace (as when cornering at a constant speed).

Three situations involving acceleration

• A car pulling away from a green traffic light: it accelerates; its speed increases.
• A car braking before a stop sign: it decelerates; its speed decreases (this is known as negative acceleration, or the acceleration vector is opposite to the direction of motion).
• A merry-go-round turning at a constant speed: the value of the speed does not change, but its direction is constantly changing, so there is indeed acceleration (towards the centre).

A common pitfall to avoid

Many pupils think that ‘constant speed’ means ‘zero acceleration’. This is only true if the path is a straight line. As soon as there is a bend, even at constant speed, there is acceleration.

Unit of acceleration

Acceleration is measured in metres per second squared (m/s²). This means ‘by how many m/s the speed changes every second’.

Situation	Initial speed	Final speed	Duration	Acceleration
Car pulling away	0 m/s	20 m/s	10 s	2 m/s²
Free fall (approx.)	0 m/s	9.8 m/s	1 s	9.8 m/s²

Remember that acceleration is a quantity that relates speed and time: it answers the question “how fast is the speed changing?”.