A Galilean cannon is a device that demonstrates conservation of linear momentum. It comprises a stack of balls, starting with a large, heavy ball at the base of the stack and progresses up to a small, lightweight ball at the top. The basic idea is that this stack of balls can be dropped to the ground and almost all of the kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 in the lower balls will be transferred to the topmost ball - which will rebound to many times the height from which it was dropped. At first sight, the behavior seems highly counter-intuitive, but in fact is precisely what conservation of momentum requires. The principal difficulty is in keeping the configuration of the balls stable during the initial drop. Early descriptions involve some sort of tube in which the balls are constrained.

A modern version of the Galilean cannon was sold by Edmund Scientific Corporation

Edmund Scientific Corporation

Edmund Scientific Corporation was a company based in Barrington, New Jersey that specialized in supplying surplus optics and other items via its mail order catalog and Factory Store. During four decades from the 1940s to the 1970s Edmund Scientific was virtually unique in its offerings to...

 and is still sold as the Astroblaster In this device, a heavy wire is threaded through all of the balls to keep them accurately aligned - but the principle is the same.

It is possible to demonstrate the principle more simply with just two balls (a basketball

Basketball

Basketball is a team sport in which two teams of five players try to score points by throwing or "shooting" a ball through the top of a basketball hoop while following a set of rules...

 and a tennis ball

Tennis ball

A tennis ball is a ball designed for the sport of tennis,approximately 6.7 cm in diameter. Tennis balls are generally bright green, but in recreational play can be virtually any color. Tennis balls are covered in a fibrous fluffy felt which modifies their aerodynamic properties...

, for example). Balance the tennis ball on top of the basketball and drop the pair to the ground. The tennis ball will rebound to many times the height from which it was released.

Contrary to what one might expect from the name, the device was not invented by Galileo Galilei

Galileo Galilei

Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 - the word "Galilean" referring to the region of Galilee

Galilee

Galilee , is a large region in northern Israel which overlaps with much of the administrative North District of the country. Traditionally divided into Upper Galilee , Lower Galilee , and Western Galilee , extending from Dan to the north, at the base of Mount Hermon, along Mount Lebanon to the...

. However, the reason for this association remains unknown.