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Momentum & Collisions · Physics

Isolated Systems & External Forces

Momentum is conserved only when external effects don’t add net impulse to your system.

This topic includes

Subtopics to master

Learn how to choose a system correctly, and how external forces/impulses change momentum.

Setup
Identifying System Boundaries
The “system” is what you choose to analyze; the boundary determines what counts as external.
  • What to include
  • Why it matters
  • Collision examples
Start subtopicMost important step.
Internal
Internal Force Cancellation
Internal forces occur in action–reaction pairs and cancel in the total momentum balance.
  • Pairs act on different objects
  • Sum over the system
  • Common pitfalls
Start subtopicClean logic.
External
Effects of External Forces
External forces change system momentum through external impulse: \(\Delta\vec P=\int \vec F_{\text{ext}} dt\).
  • Impulse viewpoint
  • Short vs long times
  • When you can ignore it
Start subtopicKnow the test.
Failure cases
When Momentum Is Not Conserved
If external impulse is not negligible, momentum changes and conservation won’t hold.
  • Friction over long times
  • External pushes
  • Rocket thrust (preview)
Start subtopicSpot the trap.
Practice
Practice & Exercises
Train system selection and external-impulse reasoning.
  • “Is momentum conserved?” drills
  • Boundary-selection mini cases
  • Impulse estimates
  • Friction vs collision comparisons
  • Quick conceptual checks
Open practiceGet consistent fast.