Newton's Cradle
Five swinging balls trade momentum down the line in the classic desk-toy demonstration of conservation laws.
Start building ↓The build
Build the frame
Make a rectangular frame the balls can hang from in a row.
Hang the balls
Suspend each ball by two lines so they sit touching in a straight line.
Level them
Adjust lengths until all balls hang at the same height, just kissing.
Lift and release
Pull one end ball and let it strike — one flies off the far side.
When the swinging ball hits the row, momentum and kinetic energy pass through the stationary balls and emerge in the last one, which swings out alone.
A closer look
Both momentum and kinetic energy are conserved in the near-elastic steel collisions, which is why lifting one ball ejects exactly one.
Variables to test
- 1 Lift two balls — how many fly off the other side?
- 2 Use a softer material; what happens to the energy?
More Physics
Homopolar Motor
A single AA battery, a magnet and a copper-wire loop spin into the simplest electric motor that actually works.
Switchable Electromagnet
Coil insulated wire around an iron bolt and a battery turns it into a magnet you can switch on, off, and strengthen at will.
Pendulum Timer
A swinging mass keeps remarkably steady time — build one and discover what really sets its rhythm.