Newtons Cradle, Balance Ball

Newton's cradle, named after Sir Isaac Newton, is a device that demonstrates conservation of momentum and energy via a series of swinging spheres. When one on the end is lifted and released, it strikes the stationary spheres; a force is transmitted through the stationary spheres and pushes the last one upward. The device is also known as Newton's balls or Executive Ball Clicker.


A typical Newton's cradle consists of a series of identically sized metal balls suspended in a metal frame so that they are just touching each other at rest. Each ball is attached to the frame by two wires of equal length angled away from each other. This restricts the pendulums' movements to the same plane.


Newton's cradle with two balls of equal weight and perfectly efficient elasticity. The left ball is pulled away and let go. Neglecting the energy losses, the left ball strikes the right ball, transferring all the velocity to the right ball. Because they are the same weight, the same velocity indicates all the momentum and energy are also transferred. The kinetic energy, as determined by the velocity, is converted to potential energy as it reaches the same height as the initial ball and the cycle repeats.

If one ball is pulled away and is let to fall, it strikes the first ball in the series and comes to nearly a dead stop. The ball on the opposite side acquires most of the velocity and almost instantly swings in an arc almost as high as the release height of the last ball. This shows that the final ball receives most of the energy and momentum that was in the first ball. The impact produces a compression wave that propagates through the intermediate balls. Any efficiently elastic material such as steel will do this as long as the kinetic energy is temporarily stored as potential energy in the compression of the material rather than being lost as heat.

An idealized Newton's cradle with five balls when there are no energy losses and there is always a small separation between the balls, except for when a pair is colliding.

With two balls dropped, exactly two balls on the opposite side swing out and back. With three balls dropped, three balls will swing back and forth, with the central ball appearing to swing without interruption.

Newton's cradle three-ball swing in a five-ball system. The central ball swings without any apparent interruption.

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