Is this formula given in the exam?

No. SPM Physics papers provide no formula sheet, so this must be recalled.

What does a negative acceleration mean?

It means the object is decelerating, that is, its velocity is decreasing. The acceleration points opposite to the direction of motion. A value such as a = −4 m s⁻² is perfectly valid.

Is acceleration the same as velocity?

No. Velocity is how fast something moves; acceleration is how quickly that velocity changes. An object can have a large velocity but zero acceleration if its velocity is not changing.

How is acceleration found from a graph?

On a velocity–time graph, acceleration is the gradient of the line, calculated as (change in velocity) ÷ (change in time), in m s⁻².

Acceleration Formula, SPM Physics

Acceleration

a = (v − u) / t

Not given in the exam, recall it.

What it is for

Acceleration is used in Force and Motion I (SPM Physics Form 4). Keep the SI units in every line of working. Force and Motion I

Symbols and SI units

Symbol	Meaning	SI unit
`a`	acceleration	m s⁻²
`v`	final velocity	m s⁻¹
`u`	initial velocity	m s⁻¹
`t`	time	s

Rearrangements

v = u + at
u = v − at
t = (v − u) / a

Worked example

A cyclist changes speed from u = 10 m s⁻¹ to v = 30 m s⁻¹ in t = 4 s. a = (v − u) / t = (30 − 10) / 4 = 5 m s⁻².

Common trap

Use the change in velocity (v − u), not v alone. A negative answer means deceleration.

Understanding acceleration

Acceleration is defined as the rate of change of velocity: a = (v − u) / t. Here a is the acceleration in m s⁻², v is the final velocity in m s⁻¹, u is the initial velocity in m s⁻¹, and t is the time taken in s.

The numerator (v − u) is the change in velocity, not the velocity itself. Dividing that change by the time it took gives how quickly the velocity is changing each second.

To find the final velocity: v = u + at
To find the initial velocity: u = v − at
To find the time: t = (v − u) / a

Because velocity is a vector, acceleration is a vector too, so direction and sign matter.

A worked example, step by step

A car brakes from an initial velocity u = 20 m s⁻¹ to a final velocity v = 8 m s⁻¹ in t = 3 s. Find its acceleration.

Write the formula, then substitute with units:

a = (v − u) / t
a = (8 m s⁻¹ − 20 m s⁻¹) / 3 s
a = (−12 m s⁻¹) / 3 s

So a = −4 m s⁻².

The negative sign shows the car is decelerating, that is, the acceleration acts opposite to the direction of motion. The unit m s⁻² comes from dividing m s⁻¹ by s, so the working is consistent.

Common mistakes and how this is tested

The key trap is using v alone instead of the change in velocity (v − u). Remember a negative answer means deceleration.

Other errors:

Subtracting in the wrong order, writing (u − v), which flips the sign.
Confusing acceleration with velocity or with speed.
Reading time in minutes and not converting to seconds.

This appears under define acceleration (rate of change of velocity), calculate the acceleration from a velocity–time graph gradient, and explain the meaning of a negative value. When taken from a graph, acceleration is the gradient of the velocity–time line.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026