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Louis Poinsot (; 3 January 1777 – 5 December 1859) was a French
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...
and
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...
. Poinsot was the inventor of
geometrical Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
mechanics Mechanics () is the area of physics concerned with the relationships between force, matter, and motion among Physical object, physical objects. Forces applied to objects may result in Displacement (vector), displacements, which are changes of ...
, showing how a system of forces acting on a rigid body could be resolved into a single
force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
and a couple.


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 Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...
on 3 January 1777. He attended the school of
Lycée Louis-le-Grand The Lycée Louis-le-Grand (), also referred to simply as Louis-le-Grand or by its acronym LLG, is a public Lycée (French secondary school, also known as sixth form college) located on Rue Saint-Jacques (Paris), rue Saint-Jacques in central Par ...
for secondary preparatory education for entrance to the famous
École Polytechnique (, ; also known as Polytechnique or l'X ) is a ''grande école'' located in Palaiseau, France. It specializes in science and engineering and is a founding member of the Polytechnic Institute of Paris. The school was founded in 1794 by mat ...
. In October 1794, at age 17, he took the École Polytechnique entrance exam and failed the
algebra Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...
section but was still accepted. A student there for two years, he left in 1797 to study at
École des Ponts et Chaussées École or Ecole may refer to: * an elementary school in the French educational stages normally followed by secondary education establishments (collège and lycée) * École (river), a tributary of the Seine flowing in région Île-de-France * Éco ...
to become a
civil engineer A civil engineer is a person who practices civil engineering – the application of planning, designing, constructing, maintaining, and operating infrastructure while protecting the public and environmental health, as well as improving existing i ...
. Although now on course for the practical and secure professional study of civil engineering, he discovered his true passion, abstract mathematics. Poinsot thus left the École des Ponts et Chaussées and civil engineering to become a mathematics teacher at the secondary school
Lycée Bonaparte Lycée Bonaparte is a French international school in Doha, Qatar. It includes levels ''maternelle'' (preschool) through ''lycée'' (senior high school). It is named after French Emperor Napoleon. The École Française de Doha was established in ...
in Paris, from 1804 to 1809. From there he became inspector general of the Imperial University of France. He shared the post with another famous mathematician,
Delambre Jean Baptiste Joseph, chevalier Delambre (19 September 1749 – 19 August 1822) was a French mathematician, astronomer, historian of astronomy, and geodesist. He was also director of the Paris Observatory, and author of well-known books on th ...
. On 1 November 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. By 1812 Poinsot was no longer directly teaching at École Polytechnique using substitute teacher
Reynaud Paul Reynaud (; 15 October 1878 – 21 September 1966) was a French politician and lawyer prominent in the interwar period, noted for his economic liberalism and vocal opposition to Nazi Germany. Reynaud opposed the Munich Agreement of Septembe ...
, and later
Cauchy Baron Augustin-Louis Cauchy ( , , ; ; 21 August 1789 – 23 May 1857) was a French mathematician, engineer, and physicist. He was one of the first to rigorously state and prove the key theorems of calculus (thereby creating real a ...
, 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 __NOTOC__ 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. Durin ...
from 1839 until his death. On the death of
Joseph-Louis Lagrange Joseph-Louis Lagrange (born Giuseppe Luigi LagrangiaAcadémie des Sciences The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...
. 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 The National Order of the Legion of Honour ( ), formerly the Imperial Order of the Legion of Honour (), is the highest and most prestigious French national order of merit, both military and civil. Currently consisting of five classes, it was ...
, 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 The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...
in 1858. He died in Paris on 5 December 1859. He is buried in
Pere Lachaise Cemetery Pere may refer to: *Pere, Hungary, a village in Borsod-Abaúj-Zemplén county *Pärtel-Peeter Pere (born 1985), Estonian entrepreneur, urban strategist, and politician *Rose Pere, Rangimārie Te Turuki Arikirangi Rose Pere (1937–2020), Māori New ...
in Paris. From the diary of Thomas Hirst, 20 December 1857: :... oinsotshook 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...


Legacy and tributes

The crater Poinsot on the moon is named after Poinsot. A street in Paris is called Rue Poinsot (14th Arrondissement).
Gustave Eiffel Alexandre Gustave Eiffel ( , ; Bonickhausen dit Eiffel; 15 December 1832 – 27 December 1923) was a French civil engineer. A graduate of École Centrale des Arts et Manufactures, he made his name with various bridges for the French railway net ...
included Poinsot among the 72 names of prominent French scientists on plaques around the first stage of the
Eiffel Tower The Eiffel Tower ( ; ) is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower from 1887 to 1889. Locally nicknamed "''La dame de fe ...
.


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) (translated as ''The Elements of Statics'' in 1848 by Thomas Sutton) *''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) File:Poinsot-1.jpg, ''Théorie nouvelle de la rotation des corps'' (1852) File:Poinsot-3.jpg, First page of ''Théorie nouvelle de la rotation des corps'' (1852) 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 In classical mechanics, Poinsot's construction (after Louis Poinsot) 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 ha ...
. This construction describes the motion of the
angular velocity In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...
vector \mathbf of a
rigid body In physics, a rigid body, also known as a rigid object, is a solid body in which deformation is zero or negligible, when a deforming pressure or deforming force is applied on it. The distance between any two given points on a rigid body rema ...
with one point fixed (usually its
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weight function, weighted relative position (vector), position of the d ...
). He proved that the endpoint of the vector \mathbf moves in a plane perpendicular to the
angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
(in absolute space) of the rigid body.
E. T. Whittaker Sir Edmund Taylor Whittaker (24 October 1873 – 24 March 1956) was a British mathematician, physicist, and historian of science. Whittaker was a leading mathematical scholar of the early 20th century who contributed widely to applied mathemat ...
, ''
Analytical Dynamics of Particles and Rigid Bodies ''A Treatise on the Analytical Dynamics of Particles and Rigid Bodies'' is a treatise and textbook on analytical dynamics by British mathematician Sir Edmund Taylor Whittaker. Initially published in 1904 by the Cambridge University Press, the ...
'', Cambridge UP, 4th edition, (1938), p. 152 ff.
He discovered the four Kepler-Poinsot polyhedra in 1809. Two of these had already appeared in
Kepler Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of p ...
's work of 1619, although Poinsot was unaware of this. The other two are the great icosahedron and great dodecahedron, which some people call these two the ''Poinsot solids''. In 1810 Cauchy proved, using Poinsot's definition of regular, that the enumeration of regular star polyhedra is complete. Poinsot worked on number theory studying
Diophantine equation ''Diophantine'' means pertaining to the ancient Greek mathematician Diophantus. A number of concepts bear this name: *Diophantine approximation In number theory, the study of Diophantine approximation deals with the approximation of real n ...
s. However he is best known for his work in geometry and, together with
Monge Gaspard Monge, Comte de Pelusium, Péluse (; 9 May 1746 – 28 July 1818) was a French mathematician, commonly presented as the inventor of descriptive geometry, (the mathematical basis of) technical drawing, and the father of differential geom ...
, 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 in 1846. Poinsot created the chair for Chasles which he occupied until his death in 1880.


References


Sources

* Bertrand, J. L. F. ''Discours aux funérailles de Poinsot'', Paris, 1860. * Bertrand, J. L. F. ''Notice sur Louis Poinsot'', Journal des savants (1872), pp. 405–420. * *


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 {{DEFAULTSORT:Poinsot, Louis 1777 births 1859 deaths 19th-century French mathematicians École Polytechnique alumni Burials at Père Lachaise Cemetery Lycée Louis-le-Grand alumni Members of the French Academy of Sciences Foreign members of the Royal Society Officers of the Legion of Honour Scientists from Paris