Is this formula given in the exam?

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

Is impulse the same as force?

No. Force is measured in newtons (N), while impulse is force multiplied by contact time, measured in newton-seconds (N s). Impulse tells you the total change in momentum the force produces over time.

Why does impulse have the unit N s?

Because impulse = Ft, its unit is newton × second = N s. Since it also equals mv − mu, that unit is identical to kg m s⁻¹, the two are the same physical quantity.

What if the object bounces back?

Then the final velocity points opposite to the initial velocity, so give it a negative sign. For example, u = +5 m s⁻¹ and v = −3 m s⁻¹ gives a larger momentum change than stopping alone.

Impulse Formula, SPM Physics | spmphysics.com.my

Impulse

Impulse = Ft = mv − mu

Not given in the exam, recall it.

What it is for

Impulse 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
`F`	force	N
`t`	time of contact	s
`m`	mass	kg
`v`	final velocity	m s⁻¹
`u`	initial velocity	m s⁻¹

Rearrangements

F = (mv − mu) / t
t = (mv − mu) / F

Worked example

A m = 0.5 kg ball is kicked from rest (u = 0) to v = 20 m s⁻¹. Impulse = mv − mu = (0.5 × 20) − (0.5 × 0) = 10 N s.

Common trap

Impulse equals the change in momentum. Its unit N s is the same as kg m s⁻¹. Watch the sign when the velocity reverses.

Understanding Impulse

Impulse measures the effect of a force acting over a period of time, and it equals the change in momentum it produces: Impulse = Ft = mv − mu.

Here F is the force in newtons (N), t is the contact time in seconds (s), m is the mass in kilograms (kg), v is the final velocity and u the initial velocity, both in metres per second (m s⁻¹). The product Ft and the momentum change mv − mu are two ways of writing the same quantity, so the unit N s equals kg m s⁻¹.

To find the force, rearrange to F = (mv − mu) / t; to find the contact time, use t = (mv − mu) / F. A larger impulse means a bigger change in the object’s motion.

A worked example, step by step

A trolley of mass m = 1.5 kg is pushed so that its velocity rises from u = 4 m s⁻¹ to v = 10 m s⁻¹. Find the impulse delivered to it.

Start from the definition:

Impulse = mv − mu = (1.5 kg × 10 m s⁻¹) − (1.5 kg × 4 m s⁻¹)

Impulse = 15 kg m s⁻¹ − 6 kg m s⁻¹ = 9 kg m s⁻¹ = 9 N s.

The answer is quoted to two significant figures with its unit. Notice we subtracted the two momenta rather than using the change in speed multiplied by mass at the end, both give the same result here, but writing mv − mu keeps the signs safe when velocity later reverses.

Common mistakes and how this is tested

The most common slip is to forget that impulse equals the change in momentum: use mv − mu, not just mv. Watch the sign carefully, if the object bounces back, u and v point in opposite directions and one must be negative.

Other frequent errors: quoting the unit as N instead of N s (or kg m s⁻¹); mixing up impulse with impulsive force (impulse has no division by time); and leaving velocities in km h⁻¹.

In the paper this appears under define (state what impulse is), calculate (find the impulse or the velocity change), and explain (why a longer contact time gives the same impulse with a smaller force, as in cushioning and follow-through).

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

Impulse Formula, SPM Physics