Newton's cradle or Newton's balls, named after Sir Isaac Newton is a device that demonstrates conservation of momentum and energy.
It is constructed from a series of pendulums (usually five in number) abutting one another.
Each pendulum is attached to a frame by two strings of equal length angled away from each other.
If these strings are not same in length, the balls would then be unbalanced.
This string arrangement restricts the pendulums' movements to the same plane.
The behaviour of the pendulum follows from the conservation of momentum and kinetic 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 dispersion free through the chain.
The principle demonstrated by the device, the law of impacts between bodies, was first demonstrated by the French physicist, Abbé
Mariotte in the 17th century.
Sir Isaac Newton acknowledged Mariotte's work, among that of others, in hisPrincipia.
In pedagogic settings, a cradle device 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, the conservation laws can be easily derived from Newton's second and third laws.