Louis Poinsot was a French

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 mathematician

Mathematician

A mathematician is a person whose primary area of study is the field of mathematics. Mathematicians are concerned with quantity, structure, space, and change....

 and physicist

Physicist

A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

. Poinsot was the inventor of geometrical mechanics

Mechanics

Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....

, showing how a system of forces acting on a rigid body could be resolved into a single force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 and a couple

Couple (mechanics)

In mechanics, a couple is a system of forces with a resultant moment but no resultant force. Another term for a couple is a pure moment. Its effect is to create rotation without translation, or more generally without any acceleration of the centre of mass.The resultant moment of a couple is called...

.

Life

"Everyone makes for himself a clear idea of the motion of a point, that is to say, of the motion of a corpuscle which one supposes to be infinitely small, and which one reduces by thought in some way to a mathematical point."

--Louis Poinsot, Théorie nouvelle de la rotation des corps (1834)

Louis was born in Paris on January 3, 1777. He attended the school of Lycée Louis-le-Grand

Lycée Louis-le-Grand

The Lycée Louis-le-Grand is a public secondary school located in Paris, widely regarded as one of the most rigorous in France. Formerly known as the Collège de Clermont, it was named in king Louis XIV of France's honor after he visited the school and offered his patronage.It offers both a...

 for secondary preparatory education for entrance to the famous École Polytechnique

École Polytechnique

The École Polytechnique is a state-run institution of higher education and research in Palaiseau, Essonne, France, near Paris. Polytechnique is renowned for its four year undergraduate/graduate Master's program...

. In October 1794, at age 17, he took the École Polytechnique entrance exam and failed the algebra section but was still accepted. A student there for 2 years, he left in 1797 to study at École des Ponts et Chaussées to become a civil engineer. Although now on course for the practical and secure professional study of civil engineering, he discovered his true passion, abstract mathematics. Poinsot thus gave up civil engineering and left the École des Ponts et Chaussées to become a mathematics teacher at the secondary school Lycée Bonaparte in Paris, from 1804 to 1809. From there he became inspector general of the Imperial University of France

University of France

The University of France was a highly centralized educational state organization founded by Napoleon I in 1808 and given authority not only over the individual, previously independent, universities, but also over primary and secondary education. The former individual universities were henceforth...

 where he shared a post with another famous mathematician Delambre. On November 1, 1809, Poinsot became assistant professor of analysis and mechanics at his old school the École Polytechnique.

During this period of transitions between schools and work, Poinsot had remained active in research and published a number of works on geometry, mechanics and statics so that by 1809 he had an excellent reputation (see Works).

By 1812 Poinsot was no longer directly teaching at École Polytechnique using substitute teacher Reynaud, and later Cauchy, and lost his post in 1816 when they re-organized, but he did become admissions examiner and held that for another 10 years. He also worked at the famous Bureau des Longitudes

Bureau des Longitudes

The Bureau des Longitudes is a French scientific institution, founded by decree of 25 June 1795 and charged with the improvement of nautical navigation, standardisation of time-keeping, geodesy and astronomical observation. During the 19th century, it was responsible for synchronizing clocks...

 from 1839 until his death.

On the death of Joseph-Louis Lagrange in 1813, Poinsot was elected to fill his place at the Académie des Sciences. In 1840 he became a member of the superior council of public instruction. In 1846 he was awarded an Officer of the Legion of Honor, and on the formation of the Senate in 1852 he was chosen a member of that body. Poinsot was elected Fellow of the Royal Society of London in 1858. He died in Paris on December 5, 1859.

From the diary of Thomas Hirst

Thomas Hirst

Thomas Henry Hirst was an English first-class cricketer, who played against Somerset for Yorkshire County Cricket Club, in a drawn match at The Circle, Kingston upon Hull, in 1899. He reappeared for Scotland against the Australians in 1905.Hirst was born in Lockwood, Huddersfield, Yorkshire,...

, December 20, 1857:

"..[Poinsot] shook me kindly by the hand, bid me be seated, and took his seat near me. He is now between 60 and 70 years old, with silver silken hair neatly arranged on a fine intelligent head. He is tall and thin, but although he now stoops with age and feebleness one can see that one time his figure was more than ordinarily graceful. He was loosely but neatly dressed in a large ample robe de chambre. His features are finely moulded - indeed everything about the man betokens good blood. He talks incessantly and well. I did not misunderstand a word, although he spoke always in a low tone, and now and then his voice dropped as if from weariness, but he never wandered from his point ..."

The crater Poinsot

Poinsot (crater)

Poinsot is a worn lunar crater that is located at the northern part of the Moon on the far side. It lies to the south of the walled plain Rozhdestvenskiy...

 on the Moon is named after him. A street in Paris is called Rue Poinsot (14th Arrondissement). When Gustave Eiffel

Gustave Eiffel

Alexandre Gustave Eiffel was a French structural engineer from the École Centrale Paris, an architect, an entrepreneur and a specialist of metallic structures...

 built the famous tower, he included the names of 72 prominent French scientists on plaques around the first stage, Poinsot included.

Work

"Poinsot was determined to publish only fully developed results and to present them with clarity and elegance. Consequently he left a rather limited body of work ..."

--Dictionary of Scientific Biography (see Sources)

Works include:

• Eléments de statique (1803)
• memoirs that dealt with the composition of moments and the composition of areas (1806)
• the general theory of equilibrium and of movements in systems (1806)
• polygons and polyhedra (1809)
• Theorie nouvelle de la rotation des corps (1834)

Poinsot was the inventor of geometrical mechanics, which showed how a system of forces acting on a rigid body could be resolved into a single force and a couple. Previous work done on the motion of a rigid body had been purely analytical with no visualization of the motion, and the great value of the work, as Poinsot says, it enables us to represent to ourselves the motion of a rigid body as clearly that as a moving point (Encyclopædia Britannica, 1911). In particular he devised, what is now known as, Poinsot's construction

Poinsot's construction

In classical mechanics, Poinsot's construction is a geometrical method for visualizing the torque-free motion of a rotating rigid body, that is, the motion of a rigid body on which no external forces are acting. This motion has four constants: the kinetic energy of the body and the three components...

. This construction describes the motion of the angular velocity

Angular velocity

In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...

 vector of a rigid body

Rigid body

In physics, a rigid body is an idealization of a solid body of finite size in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it...

 with one point fixed (usually its center of mass

Center of mass

In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

). He proved that the endpoint of the vector moves in a plane perpendicular to the angular momentum

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 (in absolute space) of the rigid body.

He discovered four new regular polyhedra, in 1809, two of which appear in Kepler

Johannes Kepler

Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

's work of 1619, although Poinsot was unaware of this. There are only four Kepler-Poinsot polyhedra, the other two are the great icosahedron and great dodecahedron, some people call these two the Poinsot solids. In 1810 Cauchy proved, using Poinsot's definition of regular, that the enumeration of regular polyhedra is complete. In 1990 a mistake was discovered in Poinsot's and Cauchy's definition when an internal inconsistency was discovered..

Poinsot worked on number theory studying Diophantine equation

Diophantine equation

In mathematics, a Diophantine equation is an indeterminate polynomial equation that allows the variables to be integers only. Diophantine problems have fewer equations than unknown variables and involve finding integers that work correctly for all equations...

s. However he is best known for his work in geometry and, together with Monge

Monge

Monge may refer to:* Edgard Monge , convicted of assault in Canada for failing to disclose his HIV status to his sexual partners* Gaspard Monge , mathematician and Comte de Péluse...

, regained geometry's leading role in mathematical research in France in the 19th century. Poinsot also contributed to the importance of geometry by creating a chair of advanced geometry at the Sorbonne

University of Paris

The University of Paris was a university located in Paris, France and one of the earliest to be established in Europe. It was founded in the mid 12th century, and officially recognized as a university probably between 1160 and 1250...

 in 1846. Poinsot created the chair for Chasles

Michel Chasles

Michel Floréal Chasles was a French mathematician.He was born at Épernon in France and studied at the École Polytechnique in Paris under Siméon Denis Poisson. In the War of the Sixth Coalition he was drafted to fight in the defence of Paris in 1814...

which he occupied until his death in 1880.

Sources

• J.L.F. Bertrand Discours aux funerailles de Poinsot (Paris, 1860)
• J.L.F. Bertrand, Notice sur Louis Poinsot, Journal des savants (1872), pg. 405–420
• Biography in Dictionary of Scientific Biography (New York, 1970–1990)

External links

• Biography by J J O'Connor and E F Robertson, School of Mathematics and Statistics, University of St Andrews, Scotland
• Rue Poinsot, Paris