Elastic vs Inelastic Collisions — What's the Difference?
Elastic collision
An elastic collision is one in which the total kinetic energy of the system is the same after the collision as before. The objects deform briefly and spring back, returning the stored energy.
Unit: joule (J) for the energies
Example: Two billiard balls colliding are nearly elastic: they bounce apart with almost all their kinetic energy intact — only a click of sound energy is lost.
Inelastic collision
An inelastic collision is one in which some kinetic energy is converted into other forms — heat, sound, or permanent deformation. Momentum is still conserved, but kinetic energy is not.
Unit: joule (J) for the energies
Example: A car crash is highly inelastic: the crumpling metal converts kinetic energy into heat and deformation, which is exactly what crumple zones are designed to do.
Key differences
| Aspect | Elastic collision | Inelastic collision |
|---|---|---|
| Kinetic energy | Yes — kinetic energy is conserved | No — some becomes heat, sound and deformation |
| Momentum | Always conserved | Always conserved (for an isolated system) |
| Everyday example | Billiard balls clicking together | A car crash — metal crumples, kinetic energy becomes heat |
| Outcome | Objects bounce apart | Objects may stick together (perfectly inelastic) |
| Coefficient of restitution | e = 1 | e < 1 (e = 0 when objects stick) |
'Elastic' here has nothing to do with stretchy rubber — it means kinetic energy is preserved. A collision can look violent yet be nearly elastic (two billiard balls), or look gentle yet be perfectly inelastic (a lump of clay hitting a wall). Always check the energy, not the drama.