# Newtons Cradle (DD-286)

> Newton's cradle is a row of identical metal balls hung from a frame so they just touch. Lift and release one end ball, and after the collision the ball at the far end swings out while the middle ones stay almost still. Pull two, and two swing out. The pattern follows from conservation of momentum and of kinetic energy in nearly elastic collisions: one ball leaving at the same speed is the outcome that satisfies both. Small losses to sound, heat and air resistance make the swings die down. The device is named after Isaac Newton, whose laws explain it, but it's credited to the 17th-century French scientist Edme Mariotte. The footage shows a classic desk cradle with five balls clicking back and forth.

- Department: Science
- First published: 2024-08-01
- Last updated: 2024-08-01
- Permalink: https://detailenjoyer.com/case/newtons-cradle/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-286-F01**: Releasing one ball sends one ball out the other end. _(Write-up · Exhibit B (One in, one out))_
- **DD-286-F02**: The cradle is credited to Edme Mariotte. _(Write-up · Exhibit E (Named for Newton))_

## Write-up

Lift one ball, and one ball answers. It's named for Isaac Newton but credited to Edme Mariotte.

1. **Identical balls**: A row of identical metal balls hangs so each just touches the next.
2. **One in, one out**: Release one end ball, and one ball at the far end swings out.
3. **Two laws at once**: Momentum and kinetic energy are both conserved in nearly elastic collisions.
4. **Slowly fading**: Sound, heat and air resistance gradually drain the swings.
5. **Named for Newton**: The device is named after Isaac Newton but credited to Edme Mariotte.

## Related case files

- [Golf Balling (DD-695)](https://detailenjoyer.com/case/golf-balling/): I paired it with another close look at a collision
- [Central Limits (DD-588)](https://detailenjoyer.com/case/central-limits/): I linked another classic physics demonstration device
- [You're Spinning Me Around (DD-274)](https://detailenjoyer.com/case/youre-spinning-me-around/): I linked more momentum physics

## Reel vs. record

- **Note**: 'Suspended in a straight line by wires': right. Many cradles use two wires or strings per ball in a V shape to keep them in line.
- **Note**: 'Nearly the same speed': right. Small losses to sound and heat make each swing a little lower.

## Video: Newtons Cradle: Lift One Ball, and One Ball Answers

- Duration: 21.4 s · Published: 2024-08-01
- File: https://detailenjoyer.com/assets/videos/C-Jl0niptr-.mp4

### Transcript

DID YOU KNOW? Newton's Cradle is a device that demonstrates the conservation of momentum and energy. It consists of a series of identically sized metal balls suspended in a straight line by wires. When one ball at the end is lifted and released it strikes the stationary balls transferring its momentum through the series. The ball at the opposite end then swings out with nearly the same speed as the first ball. This process repeats back and forth showcasing the principles of physics in action. NOW YOU KNOW!

## Questions & answers

**Q: How are the balls arranged?**  
A: They hang so each just touches the next. _(Source: Write-up · Exhibit A (Identical balls) · Wikipedia: Newton's cradle)_

**Q: What happens when one end ball is released?**  
A: Two in gives two out. _(Source: Write-up · Exhibit B (One in, one out) · Wikipedia: Newton's cradle)_

**Q: Which two quantities are conserved?**  
A: The collisions are nearly elastic. _(Source: Write-up · Exhibit C (Two laws at once) · Wikipedia: Newton's cradle)_

**Q: Why do the swings die down?**  
A: Each click takes a little energy. _(Source: Write-up · Exhibit D (Slowly fading) · Wikipedia: Newton's cradle)_

**Q: Who is credited with designing the device?**  
A: It's named after Newton anyway. _(Source: Write-up · Exhibit E (Named for Newton) · Wikipedia: Newton's cradle)_

## Sources

- [Wikipedia: Newton's cradle](https://en.wikipedia.org/wiki/Newton%27s_cradle)

## Cite this case file

Easterling, Marcus. "Newtons Cradle." The Detail Department (DetailEnjoyer), case file DD-286, last updated Aug 1, 2024. https://detailenjoyer.com/case/newtons-cradle/