Motion in a Straight Line
Acceleration
Acceleration is how fast velocity changes each second. Speeding up, slowing down, and even turning all count, because velocity includes direction.
Acceleration is the change in velocity divided by the time taken: a = Δv ÷ Δt. Its unit is metre per second squared (m/s²). A car going from 0 to 20 m/s in 5 seconds has an acceleration of 20 ÷ 5 = 4 m/s², so its velocity grows by 4 m/s every second.
Velocity is a vector: it has size and direction. So acceleration happens whenever either one changes. Slowing down is acceleration (pointing opposite to motion). Turning at a constant speed is also acceleration, because the direction, and so the velocity, is changing.
Where students slip up
The common mistake is thinking acceleration only means speeding up. It does not. A braking car is accelerating (negative, opposite to its motion). A car going round a bend at steady speed is accelerating too, because its direction keeps changing. Any change in velocity is acceleration.
Hold onto this
Any change in velocity, in speed or in direction, is acceleration.
Why it helps in the exam: watch for uniform circular motion: constant speed but constant acceleration towards the centre. And read the sign, negative acceleration means slowing down, not zero acceleration.
Where this connects
Acceleration is the bridge between motion and its cause. The moment you ask what makes something accelerate, you arrive at force: Newton's second law says a = F ÷ m. Motion graphs and force problems are the same idea from two sides.
Quick check
A car moves around a circular track at a constant speed of 10 m/s. Is it accelerating?
- ANo, because speed is constant
- BYes, because its direction keeps changing
- COnly if it speeds up
- DOnly at the start