Two carts on a frictionless track. Momentum is always conserved. Kinetic energy is conserved only in an elastic collision — an inelastic collision loses KE (the carts stick together).
| before | after | |
|---|---|---|
| v₁ (m/s) | ||
| v₂ (m/s) | ||
| p total (kg·m/s) | ||
| KE total (J) |
Try: equal masses, elastic, with v₂ = 0 — the carts swap velocities. Switch to inelastic and the same setup loses half the KE to deformation/heat.