Aristotle's wheel paradox
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Aristotle's wheel paradox is a
paradox A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...
or problem appearing in the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
work ''
Mechanica ''Mechanica'' ( la, Mechanica sive motus scientia analytice exposita; 1736) is a two-volume work published by mathematician Leonhard Euler which describes analytically the mathematics governing movement. Euler both developed the techniques of ...
'', traditionally attributed to
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
. It states as follows: A wheel is depicted in two-dimensional space as two
circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is cons ...
s. Its larger, outer circle is tangential to a horizontal surface (e.g. a road that it rolls on), while the smaller, inner one has the same center and is rigidly affixed to the larger. (The smaller circle could be the bead of a tire, the rim it is mounted upon, or the axle.) Assuming the larger circle rolls without slipping (or skidding) for one full revolution, the distances moved by both circles' circumferences are the same. The distance travelled by the larger circle is equal to its
circumference In geometry, the circumference (from Latin ''circumferens'', meaning "carrying around") is the perimeter of a circle or ellipse. That is, the circumference would be the arc length of the circle, as if it were opened up and straightened out ...
, but for the smaller it is greater than its circumference, thereby creating a paradox. The paradox is not limited to wheels: other things depicted in two dimensions display the same behavior such as a roll of tape, or a typical round bottle or jar rolled on its side (the smaller circle would be the mouth or neck of the jar or bottle). In an alternative version of the problem, the smaller circle, rather than the larger one, is in contact with the horizontal surface. Examples include a typical train wheel, which has a flange, or a barbell straddling a bench. American educator and philosopher,
Israel Drabkin Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
, called these Case II versions of the paradox, and a similar, but unidentical, analysis applies.


History of the paradox


In antiquity

In antiquity, the wheel problem was described in the Aristotelian ''
Mechanica ''Mechanica'' ( la, Mechanica sive motus scientia analytice exposita; 1736) is a two-volume work published by mathematician Leonhard Euler which describes analytically the mathematics governing movement. Euler both developed the techniques of ...
'', as well as in the ''Mechanica'' of
Hero of Alexandria Hero of Alexandria (; grc-gre, Ἥρων ὁ Ἀλεξανδρεύς, ''Heron ho Alexandreus'', also known as Heron of Alexandria ; 60 AD) was a Greek mathematician and engineer who was active in his native city of Alexandria, Roman Egypt. H ...
. In the former it appears as "Problem 24", where the description of the wheel is given as follows:
For let there be a larger circle ΔZΓ a smaller EHB, and A at the centre of both; let ZI be the line which the greater unrolls on its own, and HK that which the smaller unrolls on its own, equal to ZΛ. When I move the smaller circle, I move the same centre, that is A; let the larger be attached to it. When AB becomes perpendicular to HK, at the same time AΓ becomes perpendicular to ZΛ, so that it will always have completed an equal distance, namely HK for the circumference HB, and ZΛ for ZΓ. If the quarter unrolls an equal distance, it is clear that the whole circle will unroll an equal distance to the whole circle so that when the line BH comes to K, the circumference ZΓ will be ZΛ, and the whole circle will be unrolled. In the same way, when I move the large circle, fitting the small one to it, their centre being the same, AB will be perpendicular and at right angles simultaneously with AΓ, the latter to ZI, the former to HΘ. So that, when the one will have completed a line equal to HΘ, and the other to ZI, and ZA becomes again perpendicular to ZΛ, and HA to HK, so that they will be as in the beginning at Θ and I.
The problem is then stated:
Now since there is no stopping of the greater for the smaller so that it
he greater He or HE may refer to: Language * He (pronoun), an English pronoun * He (kana), the romanization of the Japanese kana へ * He (letter), the fifth letter of many Semitic alphabets * He (Cyrillic), a letter of the Cyrillic script called ''He'' ...
remains for an interval of time at the same point, and since the smaller does not leap over any point, it is strange that the greater traverses a path equal to that of the smaller, and again that the smaller traverses a path equal to that of the larger. Furthermore, it is remarkable that, though in each case there is only one movement, the center that is moved in one case rolls a great distance and in the other a smaller distance.


In the Scientific Revolution

The mathematician
Gerolamo Cardano Gerolamo Cardano (; also Girolamo or Geronimo; french: link=no, Jérôme Cardan; la, Hieronymus Cardanus; 24 September 1501– 21 September 1576) was an Italian polymath, whose interests and proficiencies ranged through those of mathematician, ...
discusses the problem of the wheel in his 1570 ''Opus novum de proportionibus numerorum'', taking issue with the presumption of its analysis in terms of motion.
Mersenne Marin Mersenne, OM (also known as Marinus Mersennus or ''le Père'' Mersenne; ; 8 September 1588 – 1 September 1648) was a French polymath whose works touched a wide variety of fields. He is perhaps best known today among mathematicians for ...
further discussed it in his 1623 ''Quaestiones Celeberrimae in Genesim'', where he suggests that the problem can be analysed by a process of expansion and contraction of the two circles. But Mersenne remained unsatisfied with his understanding, writing:
Indeed I have never been able to discover, and I do not think any one else has been able to discover whether the smaller circle touches the same point twice, or proceeds by leaps and sliding.
In his ''
Two New Sciences The ''Discourses and Mathematical Demonstrations Relating to Two New Sciences'' ( it, Discorsi e dimostrazioni matematiche intorno a due nuove scienze ) published in 1638 was Galileo Galilei's final book and a scientific testament covering muc ...
'',
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
uses the wheel problem to argue for a certain kind of
atomism Atomism (from Greek , ''atomon'', i.e. "uncuttable, indivisible") is a natural philosophy proposing that the physical universe is composed of fundamental indivisible components known as atoms. References to the concept of atomism and its atoms ...
. He begins his analysis by considering a pair of concentric
hexagon In geometry, a hexagon (from Ancient Greek, Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple polygon, simple (non-self-intersecting) hexagon is 720°. Regular hexa ...
s, as opposed to circles. Imagining this hexagon "rolling" on a surface, Galileo notices that the inner hexagon "jumps" a little space with each roll of the outer onto a new face. He then imagines what would happen to the limit as the number of faces on a polygon becomes very large, and finds that the little space that is "jumped" by the inner polygon becomes smaller and smaller. He writes:
Therefore a larger polygon having a thousand sides passes over and measures a straight line equal to its perimeter, while at the same time the smaller one passes an approximately equal line, but one interruptedly composed of a thousand little particles equal to its thousand sides with a thousand little void spaces interposed — for we may call these "void" in relation to the thousand linelets touched by the sides of the polygon.
Since a circle is just the limit in which the number of faces on the polygon becomes infinite, Galileo finds that Aristotle's wheel contains material that is filled with infinitesimal spaces or "voids", and that "the interposed voids are not quantified, but are infinitely many". This leads him to conclude that a belief in atoms – in the sense that matter is "composed of infinitely many unquantifiable atoms" – is sufficient to explain the phenomenon.
Gilles de Roberval Gilles Personne de Roberval (August 10, 1602 – October 27, 1675), French mathematician, was born at Roberval near Beauvais, France. His name was originally Gilles Personne or Gilles Personier, with Roberval the place of his birth. Biography ...
(1602–1675) is also associated with this analysis.


In the 19th century

Bernard Bolzano Bernard Bolzano (, ; ; ; born Bernardus Placidus Johann Gonzal Nepomuk Bolzano; 5 October 1781 – 18 December 1848) was a Bohemian mathematician, logician, philosopher, theologian and Catholic priest of Italian extraction, also known for his liber ...
discussed Aristotle's wheel in ''
The Paradoxes of the Infinite ''Paradoxes of the Infinite'' (German title: ''Paradoxien des Unendlichen'') is a mathematical work by Bernard Bolzano on the theory of sets. It was published by a friend and student, František Přihonský, in 1851, three years after Bolzano's d ...
'' (1851), a book that influenced
Georg Cantor Georg Ferdinand Ludwig Philipp Cantor ( , ;  – January 6, 1918) was a German mathematician. He played a pivotal role in the creation of set theory, which has become a fundamental theory in mathematics. Cantor established the importance of ...
and subsequent thinkers about the mathematics of infinity. Bolzano observes that there is a
bijection In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other s ...
between the points of any two similar arcs, which can be implemented by drawing a radius, remarking that the history of this apparently paradoxical fact dates back to Aristotle.


In the 20th century

The author of ''Mathematical Fallacies and Paradoxes'' uses a dime glued to a half-dollar (representing smaller and larger circles, respectively) with their centers aligned and both fixed to an axle, as a model for the paradox. He writes:
This is the solution, then, or the key to it. Although you are careful not to let the half-dollar slip on the tabletop, the “point” tracing the line segment at the foot of the dime is both rotating and slipping all the time. It is slipping with respect to the tabletop. Since the dime does not touch the table top, you do not notice the slipping. If you can roll the half-dollar along the table and at the same time roll the dime (or better yet the axle) along a block of wood, you can actually observe the slipping. If you have ever parked too close to the curb, you have noticed the screech made by your hubcap as it slips (and rolls) on the curb while your tire merely rolls on the pavement. The smaller the small circle relative to the large circle, the more the small one slips. Of course the center of the two circles does not rotate at all, so it slides the whole way.
Alternatively, one can reject the assumption that the smaller circle is independent of the larger circle. Imagine a tire as the larger circle, and imagine the smaller circle as the interior circumference of the tire and not as the rim. The movement of the inner circle is dependent on the larger circle. Thus its movement from any point to another can be calculated by using an inverse of their ratio.


Analysis and solutions

The paradox is that the smaller inner circle moves 2π''R'', the circumference of the larger outer circle with radius ''R'', rather than its own circumference. If the inner circle were rolled separately, it would move 2π''r'', its own circumference with radius ''r''. The inner circle is not separate but rigidly connected to the larger. So 2π''r'' is a
red herring A red herring is a figurative expression referring to a logical fallacy in which a clue or piece of information is or is intended to be misleading, or distracting from the actual question. Red herring may also refer to: Animals * Red herring (fis ...
. The inner circle's center and radius are both relevant, but its circumference is not.


First solution

If the smaller circle depends on the larger one (Case I), the larger circle's motion forces the smaller to traverse the larger’s circumference. If the larger circle depends on the smaller one (Case II), then the smaller circle's motion forces the larger circle to traverse the smaller circle’s circumference. This is the simplest solution.


Second solution

This solution considers the transition from the starting to ending positions. Let Pb be a point on the bigger circle and Ps be a point on the smaller circle, both on the same radius. For convenience, assume they are both directly below the center, analogous to both hands of a clock pointing towards six. Both Pb and Ps travel in a
cycloid In geometry, a cycloid is the curve traced by a point on a circle as it rolls along a straight line without slipping. A cycloid is a specific form of trochoid and is an example of a roulette, a curve generated by a curve rolling on another curve ...
path as they roll together one revolution.The two paths are pictured here: http://mathworld.wolfram.com/Cycloid.html and http://mathworld.wolfram.com/CurtateCycloid.html While each travels 2π''R'' horizontally from start to end, Ps's cycloid path is shorter and more efficient than Pb's. Pb travels farther above and farther below the center's path – the only straight one – than does Ps. If Pb and Ps were anywhere else on their respective circles, the curved paths would be the same length. Summarizing, the smaller circle moves horizontally 2π''R'' because any point on the smaller circle travels a shorter, and thus more direct path than any point on the larger circle.


Third solution

This solution only compares the starting and ending positions. The larger circle and the smaller circle have the same center. If said center is moved, both circles move the same distance, which is a necessary property of
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
and equals 2π''R'' in the experiment. Also, every other point on both circles has the same position relative to the center before and after rolling one revolution (or any other integer count of revolutions). For a wheel with multiple concentric inner circles, each circle's translation movement is identical because all have an identical center. This further proves the circumference of any inner circle is entirely irrelevant (Case 1).


See also

*
Trochoid In geometry, a trochoid () is a roulette curve formed by a circle rolling along a line. It is the curve traced out by a point fixed to a circle (where the point may be on, inside, or outside the circle) as it rolls along a straight line. If the ...
*
Brachistochrone curve In physics and mathematics, a brachistochrone curve (), or curve of fastest descent, is the one lying on the plane between a point ''A'' and a lower point ''B'', where ''B'' is not directly below ''A'', on which a bead slides frictionlessly under ...


References


Further reading


''Rota Aristotelica'', The Archimedes Project, Digital Research Library
* {{Aristotelianism Philosophy of Aristotle Circles Physical paradoxes Wheels