Nano PHYSICS . MECHANICS . MOMENTUM AND COLLISIONS . SYSTEM BOUNDARIES AND EXTERNAL FORCES

Internal Force Cancellation

Internal forces occur in action-reaction pairs and cancel in the total momentum balance for the whole system. Learn why pairs act on different objects and how to sum over the system.

Physics · Mechanics · Momentum and Collisions · System Boundaries and External Forces · Internal Force Cancellation
Access for this nano-lesson
Unsigned visitors can show & copy prompts for Steps 1-3. Signed-in free accounts can also Run with AI for Steps 1-2. Paid accounts unlock everything (Steps 1-6 + Help prompts + AI).
Steps 1-3 Free Steps 4-6 Paid
STEP 1
Orient: internal force cancellation
Free
Establish system boundaries as the key to internal and external force classification.
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Goal: define system boundaries and explain why they control what counts as external.
STEP 2
Conceptual grounding: build the system-boundary model
Free
Build a workflow for systems, interaction intervals, internal forces, external impulses, and conservation decisions.
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Goal: classify forces and impulses as internal or external for a chosen system.
STEP 3
Real-world connection: interpret internal and external effects
Free
Apply boundary reasoning to collisions, push-offs, walls, Earth interactions, friction, and external pushes.
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Goal: explain internal cancellation, external momentum change, short-time approximations, and non-conservation cases.
STEP 4
Check your understanding: diagnostic mini-quiz
Paid
Attempt each question before revealing the system-boundary reasoning.
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Goal: identify and correct boundary, internal-force, external-impulse, and short-time errors before they become calculation errors.
STEP 5
Practice: solve increasingly rich boundary and impulse situations
Paid
Apply the model to situations that grow in complexity.
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Goal: write concise, physically justified system-boundary and external-impulse explanations.
STEP 6
Summary & reflection + explorations
Paid
Consolidate the model and explore changing system boundaries, external impulses, and conservation conclusions.
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🔒 ▶ Explore interactive simulation
Goal: carry the model forward into momentum conservation, collisions, explosions, recoil, and rocket-style systems.