Explicit physics instruction
Angular momentum reasoning links mass distribution, rotational speed, and external torque.
Use both the conservation model and an explicit system boundary; do not invoke conservation automatically.
Conserved angular momentum does not mean angular speed is constant; I can change.
Core relation: L=Iω and ΔL=τ_extΔt.
Worked example
A rotating system has I=4 kg·m² and ω=2 rad/s, so L=8 kg·m²/s. If I decreases to 2 kg·m² with negligible external torque, ω rises to 4 rad/s.
Guided question
Explain why a skater-style model speeds up when mass moves inward without claiming angular momentum was created.
Independent work
- Solve three angular-momentum or torque-time problems.
- Declare the system and external-torque assumption.
- Record one unit/assumption check.
Spiral retrieval bridge
- From Week 20: what changes angular momentum?
- From Week 15: state the external-impulse condition for linear momentum conservation.
- From Week 19: relate angular and linear speed.
Misconception check
Conserved angular momentum does not mean angular speed is constant; I can change.
Formative check
If I is halved with L conserved, what happens to ω?
Reveal response
It doubles.
Mentor evidence conference
Ask Archaemenes to audit the system boundary, moment-of-inertia reasoning, and external-torque claim.
Teacher guidance
Foundation: Compute L=Iω.
Core: Solve conservation and torque-time changes.
Extended: Evaluate conservation residuals with uncertainty.