What this covers
This standard sits within Force and Motion I. In a one-to-one lesson we make sure the idea is clear first, then move straight to applying it in the exact way SPM asks, with correct units and full working.
Formulas you may need
How it is examined
It can appear in Paper 1 (objective) and Paper 2 (structured), and where an experiment applies, in Paper 3. We do not predict which questions appear; we prepare the technique for all of them.
A common mistake
How to study it
Learn the definition precisely, practise one or two SPM-style questions with full working, and link it to the rest of Force and Motion I. If it keeps costing marks, a one-to-one lesson fixes exactly that.
Force and Motion I · Formulas · Exam Papers
What you need to know
Impulse is defined as the product of a force and the time for which it acts, Ft, and is equal to the change in momentum it produces, mv − mu, where m is mass, u is initial velocity and v is final velocity. Since impulse equals a change in momentum, its unit is the newton second (N s), which is equivalent to kg m s⁻¹.
When a force varies or acts for a very short time, it is often called an impulsive force, and it is calculated by rearranging the impulse relationship: F = (mv − mu) / t, giving the average force in newtons (N) over the contact time t.
For a given change in momentum, the impulsive force is inversely proportional to the time of contact: increasing the contact time reduces the impulsive force, while decreasing the contact time increases it. This principle explains why padded surfaces, airbags, and bending the knees on landing reduce injury, since extending the time over which momentum changes lowers the peak force experienced.
Conversely, a short, sharp contact time, such as a hammer striking a nail, produces a very large impulsive force from a relatively small change in momentum.
Worked example
Question: A ball of mass 0.5 kg travelling at 12 m s⁻¹ is brought to rest by a goalkeeper in 0.2 s. Calculate the impulse delivered to the ball and the average impulsive force exerted by the goalkeeper.
Given: m = 0.5 kg, u = 12 m s⁻¹, v = 0 m s⁻¹ (brought to rest), t = 0.2 s. Impulse = change in momentum = mv − mu = (0.5 kg × 0 m s⁻¹) − (0.5 kg × 12 m s⁻¹) = 0 kg m s⁻¹ − 6 kg m s⁻¹ = −6 kg m s⁻¹, so the magnitude of the impulse is 6 N s.
The impulsive force is F = (mv − mu) / t = −6 kg m s⁻¹ ÷ 0.2 s = −30 N, so the magnitude of the average force is 30 N, acting opposite to the ball's original direction of motion.
How it is examined
Paper 1 objective items commonly ask candidates to calculate impulse or impulsive force, or to identify the correct unit, N s or its equivalent kg m s⁻¹. Paper 2 structured and essay questions frequently require candidates to calculate impulsive force from a change in momentum and a contact time, or to explain why increasing or decreasing contact time changes the force experienced in a described situation, such as a padded helmet or a karate chop.
Paper 3 practical work may involve dropping objects onto surfaces of different softness and comparing the resulting deformation or bounce to illustrate the effect of contact time on impulsive force.
Common mistakes include forgetting that impulse and momentum change share the same unit, mixing up the impulse formula Ft with the momentum formula mv, and giving the impulsive force without considering that a longer contact time produces a smaller force for the same momentum change. Candidates should also state clearly whether an increase or a decrease in contact time is being applied, since reversing this relationship is a very common source of lost marks.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)