The short answer
Why momentum is conserved in every collision, and how elastic and inelastic collisions differ.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Momentum () is "mass in motion," and its great power is that the total momentum is conserved in any collision (with no outside force). That single rule solves most collision problems — you don't need to know the messy details of the impact.
Two key relations
Momentum: . Impulse: a force acting over time changes momentum, . In a collision with no external force, total momentum before = total momentum after.
Momentum vs kinetic energy
Momentum is conserved in EVERY collision. Kinetic energy is conserved only in ELASTIC collisions. In an inelastic collision (especially when objects stick together), some KE is lost to heat, sound and deformation — but momentum still balances. Don't assume energy is conserved.
Elastic vs inelastic
Elastic
- Kinetic energy IS conserved
- Objects bounce apart
- Idealised (e.g. billiard balls, gas molecules)
Inelastic
- Kinetic energy is NOT conserved (lost to heat/sound)
- Objects may stick together (perfectly inelastic)
- Most real-world collisions
Worked example
A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley and they stick together. What is their common velocity afterward?
Check yourself
In a perfectly inelastic collision, which quantity is conserved?
Key takeaways
- Momentum p = mv; impulse FΔt = Δp.
- Total momentum is conserved in every collision (no external force).
- Kinetic energy is conserved ONLY in elastic collisions.
- Perfectly inelastic = objects stick together; KE is lost, momentum isn't.
- Set total momentum before = total momentum after, and solve.
Practise this with real GAMSAT-style questions
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