# Instructor guide: Collision momentum and energy audit

Course: Mechanics and conservation laws

Suggested time: 45–55 minutes

## Learning target

Learners use a declared system boundary to distinguish momentum conservation from kinetic-energy conservation.

## Prepare

- Review signed velocity and system boundaries.
- Prepare one equal-mass and one unequal-mass case.
- Define an acceptable numerical residual before the activity.

## Facilitation moves

- Ask for the system boundary before any equation.
- Require signed momentum rather than speed alone.
- Have groups exchange settings and reproduce one another's ledger.

## Misconception checks

- **Momentum and kinetic energy are always both conserved.** Momentum is conserved in an isolated collision; kinetic energy is conserved only in the elastic idealization.
- **A stopped cart has no effect on the ledger.** Its mass and final motion still belong to the two-cart system.

## Accessibility and participation

- Pair arrows with signed numbers so direction is not color-dependent.
- Offer a preformatted ledger and calculator access.
- Allow keyboard entry for every model control.

## Evidence of learning

- A complete signed momentum ledger
- A correct kinetic-energy comparison
- A defended system boundary

## Extension

Introduce a measured external impulse and test whether the corrected momentum ledger closes.

## Evidence boundary

Assess the learner's reasoning only within the declared model and recorded observations. Do not upgrade a simulation result into a claim about an unmodeled physical system.