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A typical Newton's cradle with the foremost pendulum in mid swing.

The cradle in motion.

Newton's cradle is a device that demonstrates conservation of momentum and energy. It is constructed from a series of pendulums (usually 5) abutting one another. Each pendulum is attached to a frame by two strings of equal length angled away from each other.If these strings weren't the same length, the balls would wobble. This string arrangement restricts the pendulums' movements to the same plane.

The behaviour of the pendulum follows from the conservation of momentum and energy only in the case of two pendula. Indeed, if there are r pendula there are also r unknown velocities to be calculated from the initial conditions. An additional condition for the observed outcome is that a shock wave has to propagate dispersionfree through the chain.

Newton's cradle has been a popular desk toy since being invented, named and produced in 1967 by English actor Simon Prebble. At first, a wood cradle version was sold by Harrods of London and later a chrome design was created by the sculptor and future film director Richard Loncraine.

In pedagogic settings, Newton's cradle is sometimes used to present the concept of "action-reaction" (Newton's third law), with the words said to the cadence of the clacking pendulums as they execute a single cycle of swinging and clacking oscillation. This is not a very clear presentation of action-reaction. In fact, for those who have mastered high school physics the conservation laws can be easily derived from Newton's second and third laws.

The largest Newton's cradle in the world was designed by Chris Boden and is owned by The Geek Group in Kalamazoo, Michigan. It is on public display and is used for science and technology demonstrations. It consists of a set of 20 matched bowling balls each with a weight of 15 pounds (6.8 kilograms). These are suspended from cables from metal trusses in the ceiling. The cables have a length of 20 feet (6.1m) and the balls hang 3 feet (1m) off the floor.

• F. Herrmann, P. Schmälzle: A simple explanation of a well-known collision experiment, Am. J. Phys. 49, 761 (1981)
• F. Herrmann, M. Seitz: How does the ball-chain work?, Am. J. Phys. 50, 977 (1982)

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