Explicit physics instruction
Momentum can be conserved across a collision even when kinetic energy changes form. Collision type is therefore not determined by momentum conservation alone.
In an ideal elastic collision, both total momentum and total kinetic energy are conserved. In an inelastic collision, momentum may be conserved while kinetic energy decreases; in a perfectly inelastic collision, objects stick together.
Real collisions can include deformation, sound, and thermal transfer, so model labels describe limiting behavior.
Vocabulary: elastic; inelastic; perfectly inelastic; kinetic energy; deformation.
Worked example with reasoning
Two carts stick after collision. System momentum can remain conserved while some kinetic energy becomes internal energy. That makes the event perfectly inelastic in the ideal model.
Guided question
What evidence would distinguish an elastic rebound from an inelastic rebound?
Independent work
- Classify four collision scenarios.
- For each, state whether momentum and kinetic energy are conserved in the ideal model.
- Explain why sticking is a stronger condition than simply losing kinetic energy.
Misconception check
Momentum conservation does not imply kinetic-energy conservation.
Formative understanding check
What additional conserved quantity defines an ideal elastic collision?
Reveal response
Total kinetic energy, in addition to momentum.
Mentor evidence conference
Ask Archaemenes to compare momentum, energy, and impulse representations for consistency.
Teacher guidance
Foundation: Use a comparison table.
Core: Classify and justify collisions from before/after evidence.
Extended: Evaluate a real collision that is only approximately elastic.