Physics 12 · Physics · Lesson 2.3
Collisions
- Level
- Grade 12
- Reading time
- 13 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Conservation laws.
What is the difference between elastic and inelastic collisions?
Short answer
In both elastic and inelastic collisions, total momentum is conserved. The difference is kinetic energy: in an elastic collision total kinetic energy is also conserved, while in an inelastic collision some kinetic energy is converted to heat, sound, or deformation. A perfectly inelastic collision is one where the objects stick together and move as one.
Concept preview
What you will be able to do
- 01Distinguish elastic, inelastic and perfectly inelastic collisions
- 02Apply conservation of momentum to all collisions
- 03Apply conservation of kinetic energy only to elastic collisions
A collision is any brief, strong interaction between objects. The golden rule is that momentum is conserved when the net external impulse on the chosen system is negligible (the mutual collision forces are internal), so always holds. Kinetic energy is a different story: sometimes it survives, sometimes some is lost to heat, sound and deformation. Sorting collisions by what happens to kinetic energy is the whole point of this topic.
School Plan
Collisions
This lesson's full explanation, worked examples, quiz and practice are locked. Full access is on the School Plan.
Log in or start free and you will return right here.
Related topics
Frequently asked questions
- Is momentum conserved in an inelastic collision?
- Yes. Momentum is conserved in every collision without external forces; only kinetic energy is lost in an inelastic one.
- What is a perfectly inelastic collision?
- One where the colliding objects stick together and move with a single common velocity afterward.