---
title: "Elastic and Inelastic Collisions"
description: "Momentum is conserved in every collision, but kinetic energy is conserved only in an elastic one. Adding up the kinetic energy before and after is what sorts one kind from the other."
canonical: https://lightmysky.com/learn/science/elastic-and-inelastic-collisions-mt_JMwUuh1YUq
source: https://lightmysky.com/learn/science/elastic-and-inelastic-collisions-mt_JMwUuh1YUq.md
retrieved: 2026-09-12
---

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# Elastic and Inelastic Collisions

Momentum is conserved in every collision, but kinetic energy is conserved only in an elastic one. Adding up the kinetic energy before and after is what sorts one kind from the other.

Subject: Science · Area: Forces & Motion · Ages 17 to 18
Page: https://lightmysky.com/learn/science/elastic-and-inelastic-collisions-mt_JMwUuh1YUq

## Ready when they can

- Applies conservation of momentum in one dimension with signs that carry the directions
- Totals the kinetic energy before and after and classifies the collision from the comparison
- Says where the missing kinetic energy has gone in an inelastic collision

## Lesson: Smashes keep momentum, audit energy

Momentum is mass times velocity, and it carries direction. Pick one direction as positive and keep it for every object, before and after. When outside forces are tiny during the brief smash, total momentum before equals total momentum after. Inside forces cancel in pairs and never change the total.

An elastic hit keeps all its kinetic energy. The pair deform and spring back with nothing lost to heat, sound, or dents. To sort a real crash, total the kinetic energy before and after. Equal totals mean elastic, and a smaller total after means inelastic.

In the extreme case the pair stick together and share one final velocity. The lost energy has not vanished. It heated metal, made sound, and bent shapes for good. The gap between the totals is the receipt for the crash.

**Recap.** Balance momentum with signs, audit the kinetic energy, and name the crash from the gap.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Power as Force Times Velocity, and Efficiency](https://lightmysky.com/learn/science/power-as-force-times-velocity-and-efficiency-mt_2FFGLIjgV6)
- [Momentum and Its Conservation](https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_)
- [Kinetic and Gravitational Potential Stores](https://lightmysky.com/learn/science/kinetic-and-gravitational-potential-stores-mt_uD3jklaZCw)

## Opens up

- [Kinetic Theory: Pressure from Molecular Motion](https://lightmysky.com/learn/science/kinetic-theory-pressure-from-molecular-motion-mt_2ARUheICqu)
- [The Compton Effect and Photon Momentum](https://lightmysky.com/learn/science/the-compton-effect-and-photon-momentum-mt_wt7kdaof_P)