Unit 05 · Momentum & Collisions
Momentum is what is conserved when things hit each other. This unit defines momentum and impulse, establishes that the total momentum of an isolated system does not change, and uses that to analyze collisions — elastic ones where kinetic energy survives, inelastic ones where it does not. Mastery means you treat momentum as a vector, know exactly what is conserved in each kind of collision, and can predict the aftermath before the carts even touch.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Momentum & impulse | Confuses momentum with force or energy. | Computes momentum but not the impulse–momentum connection. | Relates impulse to change in momentum and explains why a longer contact time softens a collision. |
| Conservation of momentum | Does not recognize when momentum is conserved. | Applies conservation but drops the vector directions. | Applies conservation to isolated systems as a vector equation and defends why external forces would break it. |
| Elastic vs. inelastic collisions | Assumes energy is always conserved in a collision. | Knows the difference but cannot say what is conserved in each. | Distinguishes elastic from inelastic collisions and states precisely what is conserved in each. |
| Two-dimensional momentum | Cannot handle momentum off a straight line. | Splits into components inconsistently. | Resolves momentum into components and conserves each direction independently. |
| Lab technique (collision carts) | Measures one cart and ignores the other. | Measures both but does not compare total momentum before and after. | Uses collision carts and timers to measure total momentum before and after and tests conservation with uncertainty. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“When the carts stick together it’s inelastic — momentum is still conserved, but kinetic energy isn’t; some went into deforming the bumper and heat. So I use conservation of momentum to find the shared final velocity, and I do not assume the energy carried over.”
“They both keep their energy, so I just add up the speeds and that’s the answer.”
You demonstrate this unit through collision-cart labs plus short oral checks where you reason about what is conserved aloud — not a multiple-choice test. A criterion counts as mastered only when you can both take clean data and justify the physics behind it. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.