Is deceleration different from acceleration?

Deceleration is just acceleration that slows an object down. In physics it is treated as a negative acceleration — the change in velocity is negative because the object is going slower at the end than at the start. So braking is simply acceleration with a minus sign.

Can something accelerate without changing speed?

Yes. Acceleration is a change in velocity, and velocity includes direction. An object going round a circle at a steady speed is constantly accelerating because its direction keeps changing. This is why a car turning a corner at constant speed is still accelerating.

Acceleration Explained | Physics

What is acceleration?

When a sprinter explodes off the starting blocks, when a plane thunders down a runway, or when you press the brake on a bike, something is changing — not just how fast you are going, but how quickly that speed is changing. That rate of change is acceleration.

It is easy to mix up speed and acceleration, so be clear from the start: speed tells you how fast you are going now, while acceleration tells you how fast your speed is changing. A car cruising steadily at 30 m/s has a high speed but zero acceleration, because its speed is not changing. If you have met the basics in speed, distance and time, acceleration is the natural next step.

The acceleration formula

Acceleration is defined as the change in velocity divided by the time it takes:

acceleration = change in velocity ÷ time taken

In symbols: a = (v − u) ÷ t, where:

u is the starting (initial) velocity,
v is the final velocity,
t is the time taken,
a is the acceleration.

The "change in velocity" is simply final minus starting: v − u.

The units: m/s²

Acceleration's units look strange the first time you see them: metres per second per second, written m/s². Here is why. Velocity is measured in metres per second (m/s). Acceleration is how much that velocity changes each second — so it is (m/s) per second, which we write as m/s².

An acceleration of 3 m/s² means: every second, the velocity increases by 3 m/s. After 1 s the object is going 3 m/s faster, after 2 s it is 6 m/s faster, and so on.

Worked examples

Example 1 — speeding up. A motorbike accelerates from 10 m/s to 28 m/s in 6 seconds. Find its acceleration.

a = (v − u) ÷ t = (28 − 10) ÷ 6 = 18 ÷ 6 = 3 m/s²

Example 2 — slowing down (deceleration). A train brakes from 40 m/s to 10 m/s in 15 seconds. Find its acceleration.

a = (v − u) ÷ t = (10 − 40) ÷ 15 = −30 ÷ 15 = −2 m/s²

The negative sign tells you the object is slowing down. Negative acceleration is exactly what we mean by deceleration.

Example 3 — finding final velocity. A ball starts from rest (u = 0) and accelerates at 4 m/s² for 5 seconds. How fast is it going?

Rearranging: v = u + a × t = 0 + 4 × 5 = 20 m/s

Three ways to accelerate

Because acceleration is a change in velocity, and velocity includes direction, there are three ways to accelerate:

Speeding up — the velocity gets bigger.
Slowing down — the velocity gets smaller (negative acceleration).
Changing direction — even at a steady speed, turning changes the velocity.

This third case surprises many people. A car going round a roundabout at a constant 15 m/s is still accelerating, because its direction is constantly changing.

Gravity's acceleration

One acceleration is so common it has its own symbol: g, the acceleration due to gravity. Near the Earth's surface, any falling object speeds up at about 9.8 m/s² (often rounded to 10 m/s² for easy maths). This means after 1 second of falling, an object is moving at about 10 m/s; after 2 seconds, about 20 m/s — ignoring air resistance. The same idea connects to forces in Newton's laws of motion.

Why this matters

Engineers obsess over acceleration. Too much and a car crash becomes deadly; too little and a rocket never leaves the ground. Lift designers limit acceleration so passengers feel comfortable, and phone screens use tiny accelerometers — sensors that measure acceleration — to know which way you are holding them. Understanding acceleration is understanding change in motion itself.

Try it yourself! 🧪

Measure the acceleration of a falling object — safely.

You need a phone with a slow-motion camera, a soft ball (a tennis ball or rolled-up sock), a tape measure, and a wall.

Tape the tape measure vertically against a wall, with 0 at the top.
Hold the ball at the top and record a slow-motion video as you drop it. Keep your feet clear so the ball cannot land on them.
Play it back frame by frame. Note how far the ball has fallen after each small slice of time.
Work out the velocity over the first interval and over a later interval.
Use a = (v − u) ÷ t to estimate the acceleration. You should get something close to 10 m/s² — the pull of gravity.

You have just measured g with nothing but a phone and a sock. To see how a graph reveals acceleration as a slope, visit speed-time graphs.

Acceleration Explained

Key takeaways