Choose the system before applying conservation

01
The tempting model
Applying conservation without defining what's inside the system.
02
Why it feels right
The objects named in the prompt can look like the system even when a different boundary makes the conservation law usable.
03
Where it breaks · how to catch it
Your 'conserved' quantity depends on forces that are external to your chosen system.
04
The correct model
Define the system first. A force internal to one choice is external to another — that decides whether momentum is conserved.
05
Repair · see it, then test it
worked example A gun fires a bullet. Is momentum conserved?
Tempting move
No — the bullet speeds up.
Correct move
For the gun+bullet system, yes: total momentum stays zero (recoil).
Test the repair MOM-04 · transfer

A cannon fires horizontally from a cart on a nearly frictionless track. Which system has approximately conserved horizontal momentum during firing?
Projectile only Cannon only Cannon, cart, and projectile together Earth only

Make a fresh prediction before referring to the worked example.
Then practise it on the ladder
R6Two-block system: choosing the systemadds: Internal/external depends entirely on the system boundary.R26Recoil from an internal throwadds: An internal throw conserves system momentum.