Not given in the exam, recall it.
What it is for
Impulsive force 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 | impulsive force | N |
| m | mass | kg |
| v | final velocity | m s⁻¹ |
| u | initial velocity | m s⁻¹ |
| t | time of contact | s |
Rearrangements
mv − mu = Ftt = (mv − mu) / F
Worked example
Common trap
Understanding Impulsive force
Impulsive force is the large force that acts during a very short collision or contact. It is found from the rate of change of momentum: F = (mv − mu) / t.
Here F is the impulsive force in newtons (N), 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), and t is the contact time in seconds (s). The numerator mv − mu is the change in momentum.
Rearranged, the change in momentum is mv − mu = Ft, and the contact time is t = (mv − mu) / F. Because t is on the bottom, a shorter contact time produces a much larger impulsive force for the same change in momentum.
A worked example, step by step
A tennis ball of mass m = 0.06 kg is served from rest (u = 0) to v = 25 m s⁻¹, and the racket stays in contact for t = 0.005 s. Find the impulsive force.
Write the formula, then substitute with units:
F = (mv − mu) / t = ((0.06 kg × 25 m s⁻¹) − 0) / 0.005 s
F = (1.5 kg m s⁻¹) / 0.005 s = 300 N.
The answer, 300 N to two significant figures, shows how a tiny contact time turns a modest momentum change into a large force, the whole idea behind why short, sharp impacts hurt.
Common mistakes and how this is tested
The key relationship is that a shorter contact time gives a larger impulsive force, and a longer contact time reduces it, candidates often state this the wrong way round.
Other errors: forgetting to divide the momentum change by time (that would give impulse, not force); dropping the initial momentum mu when the object is not starting from rest; and not converting the contact time from milliseconds to seconds.
Examiners test this under calculate (find F, t or the velocity change), define (state what impulsive force is), and especially explain (why crumple zones, air bags, thick gloves and bending the knees on landing all lengthen t to lower F).
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)