# Collisions

Meetings where momentum is conserved. In elastic collisions kinetic energy is also kept. In inelastic ones some is lost.

Physics (university) · Part 03: Work, energy and momentum · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Billiard balls bounce (nearly elastic). Two clay balls stick (inelastic).

## Example

**Sticking carts**

Before: 2 x 3 = 6 kg m/s
After: 3 kg x v
v = 2 m/s

## In practice

A 2 kg cart at 3 m/s sticks to a 1 kg cart at rest. They move at 2 m/s together.

## Practice

### Flashcards

- **What is conserved in every collision?** Total momentum, if no outside force acts.

### Interview questions

**A 1500 kg car travelling at 15 m/s hits a stationary 1000 kg car and they lock together. Find their speed and the energy lost.**

- Use momentum conservation
- Combined mass is 2500 kg
- Compare kinetic energy before and after

## Questions people ask

### What is the difference between elastic and inelastic collisions?

In both, momentum is conserved. In an elastic collision, total kinetic energy is also conserved, as with ideal billiard balls. In an inelastic collision some kinetic energy becomes heat, sound or deformation. If objects stick together, it is perfectly inelastic and loses the most.

### How do I solve a two-object collision?

Choose a positive direction. Write total momentum before equal to total momentum after. For elastic collisions add a second equation, conservation of kinetic energy. Solve the equations for the unknown speeds. Check that kinetic energy never increases in a collision.

## Related concepts

- [Momentum](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-7)
- [Kinetic energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-2)
- [Impulse](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-8)

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