Newton's cradle online
A Newton's cradle is a row of steel balls hanging from thin strings. Lift one ball and let it fall: it stops dead, and the ball at the far end flies out. Lift two, and two fly out. This game runs a real physics simulation of every swing and every collision, and turns the desk toy into a puzzle: ring every bell, and don't crack the eggs.
How to play
- Pull and let go. Drag a ball out to the side and release it. Pull an inner ball to lift several balls at once.
- Only a swinging ball rings a bell. Your hand stops when it reaches a bell or an egg, so you have to let the cradle do the work.
- Mind the eggs. An egg sits just above where a bell can be reached. Swing too hard and it cracks.
- Count your pulls. Match the level's par for three stars. A rung bell stays lit, so on two-pull levels plan which pull rings which bell.
- Aim with the ghost. A dashed outline shows where you let go last time, so you can pull a little more or a little less.
How does a Newton's cradle work?
When the moving ball hits the row, the collision has to conserve two things at once: momentum (mass × speed) and kinetic energy. For balls of equal mass there is only one way to satisfy both: the same number of balls must leave the far end at the same speed as the ones that arrived. One ball in, one ball out. Two in, two out. If two balls came out at half speed, momentum would balance but half of the energy would vanish, which can't happen in an elastic collision.
The impact travels through the middle balls as a compression wave in a fraction of a millisecond, which is why the middle balls appear not to move at all.
Why does it slow down and stop?
Real steel balls are not perfectly elastic. Each click turns a little energy into sound and heat, the strings stretch, and air drags on the balls. That is why a ball you let go never quite returns to the height you dropped it from, and why the puzzles make you send energy across the row instead.
Different balls, different physics
- Brass is three times heavier than steel. Dropped into a steel row, it launches the last ball higher than it fell.
- Aluminium weighs a third as much. It barely nudges the row and bounces back.
- Clay absorbs the hit, so the balls behind it shuffle together instead of flying out.
- Glass shatters if it is hit too hard and leaves a gap in the row.
Frequently asked questions
Is this Newton's cradle game free?
Yes. It runs in your browser for free, with no download and no account.
Why can't I just push a ball into the bell?
Because that would skip the physics. Your hand stops at a bell or an egg; only a ball swinging on its own can ring a bell. A ball let go on one side never swings back quite as high, so a bell on that side has to be reached through the collisions.
Why does one ball come out when one ball goes in?
Because the collision has to keep both momentum and kinetic energy. With equal balls, the only way to do both is for the same number of balls to leave at the same speed as the ones that came in.
Why does a Newton's cradle eventually stop?
Every click turns a little energy into sound and heat, and air resistance slows the balls. That is also why a ball you let go never swings back quite as high.
What happens with balls of different weights?
A heavy brass ball pushes the row harder, so the last ball flies out higher than the brass ball was dropped. A light aluminium ball barely moves the row and bounces back.
How do I get three stars?
Ring every bell using no more pulls than the level's par. One extra pull gives two stars. Stars unlock the later chapters.