Momentum is conserved only when external impulse is negligible
The mistake
Applying conservation of momentum with a large external force present.
Why it feels right
'Momentum is conserved' is stated so often it feels unconditional.
How to catch it
You conserved momentum across an interval with significant external force × time.
The first principle
Δp = external impulse. Momentum of a system is conserved only when the net external impulse ≈ 0 — true for brief collisions, not over long gravity-driven intervals.
worked mini-example
Is momentum conserved for a ball during a 2 s fall?
Common wrong move
Yes — momentum is always conserved.
Correct
No — gravity delivers external impulse mgΔt, so p changes.
Test the repair MOM-01 · transfer
Make a fresh prediction before referring to the worked example.