Newton's Cradle

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, the resulting force travels through the line and pushes the last one upward. The device is also known as Newton's balls. (Do not laugh :P)

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.

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 first ball. This shows that the final ball receives most of the energy and momentum that was in the first ball. Intrigue is provided by starting more than one ball in motion. With two balls, exactly two balls on the opposite side swing out and back.The impact produces a shock 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.

While the symmetry is satisfying, why doesn't the initial ball (or balls) bounce back instead of imparting nearly all the momentum and energy to the last ball (or balls)? The simple equations used for the conservation of kinetic energy and conservation of momentum can show this is a possible solution, but they can't be used to predict the final velocities when there are 3 or more balls in a cradle because they provide only 2 equations to find the 3 or more unknowns (velocities of the balls). They give an infinite number of possible solutions if the system of balls is not examined in more detail.More than half the balls can be set in motion. For example, three out of five balls will result in the central ball swinging without any apparent interruption.

This is total AWESOMENESS!