Separate momentum conservation from kinetic-energy conservation
Three steps, the way the exam actually works: work through the lab, write down your own measurements, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own numbers will cost you the point — exactly as it would with a real reader.
Predict before you look
- Momentum is conserved when the net external force is zero. Mechanical energy is conserved when no non-conservative force does work. Are those the same condition?
- In a perfectly inelastic collision the objects stick together. Where does the missing kinetic energy go?
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you measured
These are your numbers, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Elastic trial total momentum before | kg·m/s |
|---|---|
| Elastic trial total momentum after | kg·m/s |
| Elastic trial total kinetic energy before | J |
| Elastic trial total kinetic energy after | J |
| Inelastic trial total momentum before | kg·m/s |
| Inelastic trial total momentum after | kg·m/s |
| Inelastic trial total kinetic energy before | J |
| Inelastic trial total kinetic energy after | J |
Answer the free response
The collision lab displays two conservation laws that students often blur together. (a) Using your two runs, state which quantity is conserved in both collisions and justify it with your recorded totals. (b) Explain why kinetic energy is conserved in an elastic collision but not in a perfectly inelastic collision. (c) In the perfectly inelastic case, identify where the “missing” kinetic energy goes. (d) A student sees equal total momentum before and after and concludes that the individual cart speeds must also stay the same. Evaluate that reasoning.
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