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Revision as of 07:29, 16 April 2010
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Plane (geometry): In elementary geometry, a flat surface that entirely contains all straight lines passing through two of its points. [e]
This article is about the geometrical concept. For other uses of the term Plane, please see Plane (disambiguation).
In Euclidean geometry, a plane is an abstract concept that models the common notion of a flat surface — without depressions, protrusions, holes and a boundary — that for any two of its points entirely contains the straight line joining them. Assuming a common (intuitive, physical) idea of the geometry of space, "plane" can be defined in terms of distances, orthogonality, lines, coordinates etc. In a more abstract approach (vector spaces) planes are defined as two-dimensional affine subspaces. In an axiomatic approach, basic concepts of elementary geometry, such as "point", "line" and "plane", are undefined primitives. Non-axiomatic approachDefinitionsA remarkTo define a plane is more complicated than it may seem. It is tempting to define a plane as a surface with zero curvature, where a surface is defined as a geometric object having length and breadth but no depth. However, this is not a good idea; such definitions are useless in mathematics, since they cannot be used when proving theorems. Planes are treated by elementary geometry, but the notions of surface and curvature are not elementary, they need more advanced mathematics and more sophisticated definitions. Fortunately, it is possible to define a line via more elementary notions, and this way is preferred in mathematics. Still, the definitions given below are tentative. They are criticized afterwards, see axiomatic approach. The definitions of "plane" given below may be compared with the definition of a circle as consisting of those points in a plane that are a given distance (the radius) away from a given point (the center). A circle is a set of points chosen according to their relation to some given parameters (center and radius). Similarly, a plane is a set of points chosen according to their relation to some given objects (points, lines etc). However, a circle determines its center and radius uniquely; for a plane, the situation is different. Four equivalent definitions of "plane" are given below. Any other definition is equally acceptable provided that it is equivalent to these. Note that a part of a plane is not a plane. Likewise, a line segment is not a line. Below, all points, lines and planes are situated in the space (assumed to be a three-dimensional Euclidean space), and by lines we mean straight lines. Definition via distancesLet two different points A and B be given. The set of all points C that are equally far from A and B — that is, — is a plane. This is the plane orthogonal to the line AB through the middle point of the line segment AB. Definition via right angles (orthogonality)Let two different points A and B be given. The set of all points C such that the lines AB and AC are orthogonal (that is, the angle BAC is right) is a plane. This is the plane orthogonal to the line AB through the point A. Definition via linesLet three points A, B and C be given, not lying on a line. Consider the lines DE for all points D (different from B) on the line AB and all points E (also different from B) on the line BC. The union of all these lines, together with the point B, is a plane. This is the plane through A, B and C. In other words, this plane is the set of all points F such that either F coincides with B or there exists a line through F that intersects the lines AB and BC (in distinct points). Definition via Cartesian coordinatesIn terms of Cartesian coordinates x, y, z ascribed to every point of the space, a plane is the set of points whose coordinates satisfy the linear equation
Here real numbers a, b, c and d are parameters such that at least one of a, b, c does not vanish. Some properties of planesMost basic propertiesFor any three points not situated in the same straight line there exists one and only one plane that contains these three points. If two points of a straight line lie in a plane, then every point of the line lies in that plane. If two planes have a common point then they have at least a second point in common. Every plane contains at least three points not lying in the same straight line, and the space contains at least four points not lying in a plane. Further propertiesTwo planes either do not intersect (are parallel), or intersect in a line, or coincide. A line either does not intersect a plane (is parallel to it), or intersects it in a single point, or is contained in the plane. Two lines perpendicular to the same plane are parallel to each other (or coincide). Two planes perpendicular to the same line are parallel to each other (or coincide). Axiomatic approachWhat is wrong with the definitions given above?The definitions given above assume implicitly that the 3-dimensional Euclidean space is already defined, together with (at least one of) such notions as distances, angles, straight lines, Cartesian coordinates, while planes are not defined yet. However, this situation never appears in mathematical theory. In the axiomatic approach points, lines and planes are undefined primitives. The modern approach (below) defines planes in a completely different way. How does it workAxiomatic approach is similar to chess in the following aspect. A chess piece, say a rook, cannot be defined before the whole chess game is defined, since such a phrase as "the rook moves horizontally or vertically, forward or back, through any number of unoccupied squares" makes no sense unless it is already known that "chess is played on a square board of eight rows and eight columns" etc. And conversely, the whole chess game cannot be defined before each piece is defined; the properties of the rook are an indispensable part of the rules of the game. No chess without rooks, no rooks outside chess! One must introduce the game, its pieces and their properties in a single combined definition. Likewise, Euclidean space, its points, lines, planes and their properties are introduced simultaneously in a set of 20 assumptions known as Hilbert's axioms of Euclidean geometry.[1] The "most basic properties of planes" listed above are roughly the plane-related assumptions (Hilbert's axioms), while "further properties" are the first plane-related consequences (theorems). Modern approachThe modern approach defines the three-dimensional Euclidean space more algebraically, via linear spaces and quadratic forms, namely, as a real affine space whose difference space is a three-dimensional inner product space. For further details see Affine space#Euclidean space and space (mathematics). In this approach a plane in an n-dimensional affine space (n ≥ 2) is defined as a (proper or improper) two-dimensional affine subspace. A less formal version of this approach uses points, vectors and scalar product (called also dot product or inner product) of vectors without mentioning linear and affine spaces. Optionally, Cartesian coordinates of points and vectors are used. See algebraic equations below. There, in particular, equivalence between the definition via right angles (orthogonality) and the definition via Cartesian coordinates is explained. Plane geometryPlane geometry (also called "planar geometry") is a part of solid geometry that restricts itself to a single plane ("the plane") treated as a geometric universe. In other words, plane geometry is the theory of the two-dimensional Euclidean space, while solid geometry is the theory of the three-dimensional Euclidean space. Plane geometry studies the properties of plane figures (and configurations). Plane figures in elementary geometry are sets of points, lines, line segments and sometimes curves that fall on the same plane. For example, triangles, polygons and circles. In plane geometry every figure is plane, in contrast to solid geometry. Algebraic equationsIn analytic geometry several closely related algebraic equations are known for a plane in three-dimensional Euclidean space. A few algebraic representations will be discussed. Point-normal representationOne such equation is illustrated in the figure. Point P is an arbitrary point in the plane and O (the origin) is drawn outside the plane. The point A in the plane is chosen such that vector is orthogonal to the plane. The collinear vector is a unit (length 1) vector normal (perpendicular) to the plane which is known as the normal of the plane in point A. Note that d is the distance of O to the plane. The following relation holds for an arbitrary point P in the plane (according to the definition via right angles): This equation for the plane can be rewritten in terms of coordinates with respect to a Cartesian frame with origin in O. Dropping arrows for component vectors (real triplets) that are written bold, we find with and The definition via Cartesian coordinates is thus derived from the definition via right angles. Moreover, the Hesse normal form for the plane (called after the 19th century mathematician Ludwig Otto Hesse) is obtained, it is characterized by the use of a unit-length vector rather than an arbitrary vector orthogonal to the plane. Conversely, the definition via right angles can be derived from the definition via Cartesian coordinates as follows. Given a linear equation for a plane we write It follows that Hence we find the same orthogonality relation, where f , d, and n0 are collinear. The equation may also be written in the following mnemonically convenient form which is the equation for a plane through a point A perpendicular to . Three-point representationThe figure shows a plane that by definition passes through three different points A, B, and C that are not on one line. The point P is an arbitrary point in the plane and the reference point O is again drawn outside the plane, but the case that the plane passes through O is not excluded. Referring to figure 2 we introduce the following definitions Clearly the following two non-collinear vectors belong to the plane Because a plane (an affine space), with a given fixed point as origin is a 2-dimensional linear space and two non-collinear vectors with "tails" in the origin are linearly independent, it follows that any vector in the plane can be written as a linear combination of these two non-collinear vectors. (This is also expressed as: Any vector in the plane can be decomposed into components along the two non-collinear vectors.) In particular, taking A as origin in the plane, The real numbers λ and μ specify the direction of . Hence the following equation for the position vector of the arbitrary point P in the plane: is known as the point-direction representation of the plane. This representation is equal to the three-point representation where , , and are the position vectors of the three points that define the plane. Writing for the position vector of the arbitrary point P in the plane we find that the real triplet (ξ1, ξ2, ξ3) with ξ1 + ξ1 + ξ1 = 1 forms a set of coordinates for P. The numbers {ξ1, ξ2, ξ3 | ξ1+ ξ2+ ξ3 = 1 } are known as the barycentric coordinates of P. It is trivial to go from barycentric coordinates to the "three-point representation", Beyond mathematicsIn industry, a surface plate[2] is a piece of cast iron or other appropriate material whose surface (or rather a part of it) is made as close as possible to a geometric plane (or rather a part of it, usually a square). An old method of their manufacturing is the three-plate method: three roughly flat surfaces become more and more flat when rubbing against each other: first and second; second and third; third and first; first and second again, and so on. It is possible to achieve a surface close to a plane up to 10–5 of its size. Reference
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Daniel Mietchen ; Meg Ireland; Chunbum Park |
Peter Schmitt | 5
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In Euclidean geometry, a plane is an abstract concept that models the common notion of a flat surface — without depressions, protrusions, holes and a boundary — that for any two of its points entirely contains the straight line joining them.
Assuming a common (intuitive, physical) idea of the geometry of space, "plane" can be defined in terms of distances, orthogonality, lines, coordinates etc. In a more abstract approach (vector spaces) planes are defined as two-dimensional affine subspaces. In an axiomatic approach, basic concepts of elementary geometry, such as "point", "line" and "plane", are undefined primitives.
Non-axiomatic approach
Definitions
A remark
To define a plane is more complicated than it may seem.
It is tempting to define a plane as a surface with zero curvature, where a surface is defined as a geometric object having length and breadth but no depth. However, this is not a good idea; such definitions are useless in mathematics, since they cannot be used when proving theorems. Planes are treated by elementary geometry, but the notions of surface and curvature are not elementary, they need more advanced mathematics and more sophisticated definitions. Fortunately, it is possible to define a line via more elementary notions, and this way is preferred in mathematics. Still, the definitions given below are tentative. They are criticized afterwards, see axiomatic approach.
The definitions of "plane" given below may be compared with the definition of a circle as consisting of those points in a plane that are a given distance (the radius) away from a given point (the center). A circle is a set of points chosen according to their relation to some given parameters (center and radius). Similarly, a plane is a set of points chosen according to their relation to some given objects (points, lines etc). However, a circle determines its center and radius uniquely; for a plane, the situation is different.
Four equivalent definitions of "plane" are given below. Any other definition is equally acceptable provided that it is equivalent to these. Note that a part of a plane is not a plane. Likewise, a line segment is not a line.
Below, all points, lines and planes are situated in the space (assumed to be a three-dimensional Euclidean space), and by lines we mean straight lines.
Definition via distances
Let two different points A and B be given. The set of all points C that are equally far from A and B — that is,
— is a plane.
This is the plane orthogonal to the line AB through the middle point of the line segment AB.
Definition via right angles (orthogonality)
Let two different points A and B be given. The set of all points C such that the lines AB and AC are orthogonal (that is, the angle BAC is right) is a plane.
This is the plane orthogonal to the line AB through the point A.
Definition via lines
Let three points A, B and C be given, not lying on a line. Consider the lines DE for all points D (different from B) on the line AB and all points E (also different from B) on the line BC. The union of all these lines, together with the point B, is a plane.
This is the plane through A, B and C.
In other words, this plane is the set of all points F such that either F coincides with B or there exists a line through F that intersects the lines AB and BC (in distinct points).
Definition via Cartesian coordinates
In terms of Cartesian coordinates x, y, z ascribed to every point of the space, a plane is the set of points whose coordinates satisfy the linear equation
- .
Here real numbers a, b, c and d are parameters such that at least one of a, b, c does not vanish.
Some properties of planes
Most basic properties
For any three points not situated in the same straight line there exists one and only one plane that contains these three points.
If two points of a straight line lie in a plane, then every point of the line lies in that plane.
If two planes have a common point then they have at least a second point in common.
Every plane contains at least three points not lying in the same straight line, and the space contains at least four points not lying in a plane.
Further properties
Two planes either do not intersect (are parallel), or intersect in a line, or coincide.
A line either does not intersect a plane (is parallel to it), or intersects it in a single point, or is contained in the plane.
Two lines perpendicular to the same plane are parallel to each other (or coincide).
Two planes perpendicular to the same line are parallel to each other (or coincide).
Axiomatic approach
What is wrong with the definitions given above?
The definitions given above assume implicitly that the 3-dimensional Euclidean space is already defined, together with (at least one of) such notions as distances, angles, straight lines, Cartesian coordinates, while planes are not defined yet. However, this situation never appears in mathematical theory.
In the axiomatic approach points, lines and planes are undefined primitives.
The modern approach (below) defines planes in a completely different way.
How does it work
Axiomatic approach is similar to chess in the following aspect.
A chess piece, say a rook, cannot be defined before the whole chess game is defined, since such a phrase as "the rook moves horizontally or vertically, forward or back, through any number of unoccupied squares" makes no sense unless it is already known that "chess is played on a square board of eight rows and eight columns" etc. And conversely, the whole chess game cannot be defined before each piece is defined; the properties of the rook are an indispensable part of the rules of the game. No chess without rooks, no rooks outside chess! One must introduce the game, its pieces and their properties in a single combined definition.
Likewise, Euclidean space, its points, lines, planes and their properties are introduced simultaneously in a set of 20 assumptions known as Hilbert's axioms of Euclidean geometry.[1] The "most basic properties of planes" listed above are roughly the plane-related assumptions (Hilbert's axioms), while "further properties" are the first plane-related consequences (theorems).
Modern approach
The modern approach defines the three-dimensional Euclidean space more algebraically, via linear spaces and quadratic forms, namely, as a real affine space whose difference space is a three-dimensional inner product space. For further details see Affine space#Euclidean space and space (mathematics).
In this approach a plane in an n-dimensional affine space (n ≥ 2) is defined as a (proper or improper) two-dimensional affine subspace.
A less formal version of this approach uses points, vectors and scalar product (called also dot product or inner product) of vectors without mentioning linear and affine spaces. Optionally, Cartesian coordinates of points and vectors are used. See algebraic equations below. There, in particular, equivalence between the definition via right angles (orthogonality) and the definition via Cartesian coordinates is explained.
Plane geometry
Plane geometry (also called "planar geometry") is a part of solid geometry that restricts itself to a single plane ("the plane") treated as a geometric universe. In other words, plane geometry is the theory of the two-dimensional Euclidean space, while solid geometry is the theory of the three-dimensional Euclidean space.
Plane geometry studies the properties of plane figures (and configurations). Plane figures in elementary geometry are sets of points, lines, line segments and sometimes curves that fall on the same plane. For example, triangles, polygons and circles. In plane geometry every figure is plane, in contrast to solid geometry.
Algebraic equations
In analytic geometry several closely related algebraic equations are known for a plane in three-dimensional Euclidean space. A few algebraic representations will be discussed.
Point-normal representation
One such equation is illustrated in the figure. Point P is an arbitrary point in the plane and O (the origin) is drawn outside the plane. The point A in the plane is chosen such that vector
is orthogonal to the plane. The collinear vector
is a unit (length 1) vector normal (perpendicular) to the plane which is known as the normal of the plane in point A. Note that d is the distance of O to the plane. The following relation holds for an arbitrary point P in the plane (according to the definition via right angles):
This equation for the plane can be rewritten in terms of coordinates with respect to a Cartesian frame with origin in O. Dropping arrows for component vectors (real triplets) that are written bold, we find
with
and
The definition via Cartesian coordinates is thus derived from the definition via right angles.
Moreover, the Hesse normal form for the plane (called after the 19th century mathematician Ludwig Otto Hesse) is obtained,
it is characterized by the use of a unit-length vector rather than an arbitrary vector orthogonal to the plane.
Conversely, the definition via right angles can be derived from the definition via Cartesian coordinates as follows. Given a linear equation for a plane
we write
It follows that
Hence we find the same orthogonality relation,
where f , d, and n0 are collinear. The equation may also be written in the following mnemonically convenient form
which is the equation for a plane through a point A perpendicular to .
Three-point representation
The figure shows a plane that by definition passes through three different points A, B, and C that are not on one line. The point P is an arbitrary point in the plane and the reference point O is again drawn outside the plane, but the case that the plane passes through O is not excluded. Referring to figure 2 we introduce the following definitions
Clearly the following two non-collinear vectors belong to the plane
Because a plane (an affine space), with a given fixed point as origin is a 2-dimensional linear space and two non-collinear vectors with "tails" in the origin are linearly independent, it follows that any vector in the plane can be written as a linear combination of these two non-collinear vectors. (This is also expressed as: Any vector in the plane can be decomposed into components along the two non-collinear vectors.) In particular, taking A as origin in the plane,
The real numbers λ and μ specify the direction of . Hence the following equation for the position vector of the arbitrary point P in the plane:
is known as the point-direction representation of the plane. This representation is equal to the three-point representation
where , , and are the position vectors of the three points that define the plane.
Writing for the position vector of the arbitrary point P in the plane
we find that the real triplet (ξ1, ξ2, ξ3) with ξ1 + ξ1 + ξ1 = 1 forms a set of coordinates for P. The numbers {ξ1, ξ2, ξ3 | ξ1+ ξ2+ ξ3 = 1 } are known as the barycentric coordinates of P. It is trivial to go from barycentric coordinates to the "three-point representation",
Beyond mathematics
In industry, a surface plate[2] is a piece of cast iron or other appropriate material whose surface (or rather a part of it) is made as close as possible to a geometric plane (or rather a part of it, usually a square). An old method of their manufacturing is the three-plate method: three roughly flat surfaces become more and more flat when rubbing against each other: first and second; second and third; third and first; first and second again, and so on. It is possible to achieve a surface close to a plane up to 10–5 of its size.
Reference
- ↑ D. Hilbert, Grundlagen der Geometrie, B. G. Teubner, Leipzig (1899) 2nd German edition
- ↑ Miller, Jimmie A. (5 October 2004). Surface Plate. University of North Carolina. Retrieved on 29 October 2013.
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- Origins and architecture of the Taj Mahal [r]: Add brief definition or description - 11 October 2007
- Fossilization (palaeontology) [r]: Add brief definition or description - 4 October 2007
- Cradle of Humankind [r]: Add brief definition or description - 27 September 2007
- John Adams [r]: Add brief definition or description - 20 September 2007
- Quakers [r]: Add brief definition or description - 13 September 2007
- Scarborough Castle [r]: Add brief definition or description - 6 September 2007
- Jane Addams [r]: Add brief definition or description - 30 August 2007
- Epidemiology [r]: Add brief definition or description - 23 August 2007
- Gay community [r]: Add brief definition or description - 16 August 2007
- Edward I [r]: Add brief definition or description - 9 August 2007
Rules and Procedure
Rules
- The primary criterion of eligibility for a new draft is that it must have been ranked as a status 1 or 2 (developed or developing), as documented in the History of the article's Metadate template, no more than one month before the date of the next selection (currently every Thursday).
- Any Citizen may nominate a draft.
- No Citizen may have nominated more than one article listed under "current nominees" at a time.
- The article's nominator is indicated simply by the first name in the list of votes (see below).
- At least for now--while the project is still small--you may nominate and vote for drafts of which you are a main author.
- An article can be the New Draft of the Week only once. Nominated articles that have won this honor should be removed from the list and added to the list of previous winners.
- Comments on nominations should be made on the article's talk page.
- Any draft will be deleted when it is past its "last date eligible". Don't worry if this happens to your article; consider nominating it as the Article of the Week.
- If an editor believes that a nominee in his or her area of expertise is ineligible (perhaps due to obvious and embarrassing problems) he or she may remove the draft from consideration. The editor must indicate the reasons why he has done so on the nominated article's talk page.
Nomination
See above section "Add New Nominees Here".
Voting
- To vote, add your name and date in the Supporters column next to an article title, after other supporters for that article, by signing
<br />~~~~
. (The date is necessary so that we can determine when the last vote was added.) Your vote is alloted a score of 1. - Add your name in the Specialist supporters column only if you are an editor who is an expert about the topic in question. Your vote is alloted a score of 1 for articles that you created and 2 for articles that you did not create.
- You may vote for as many articles as you wish, and each vote counts separately, but you can only nominate one at a time; see above. You could, theoretically, vote for every nominated article on the page, but this would be pointless.
Ranking
- The list of articles is sorted by number of votes first, then alphabetically.
- Admins should make sure that the votes are correctly tallied, but anyone may do this. Note that "Specialist Votes" are worth 3 points.
Updating
- Each Thursday, one of the admins listed below should move the winning article to the Current Winner section of this page, announce the winner on Citizendium-L and update the "previous winning drafts" section accordingly.
- The winning article will be the article at the top of the list (ie the one with the most votes).
- In the event of two or more having the same number of votes :
- The article with the most specialist supporters is used. Should this fail to produce a winner, the article appearing first by English alphabetical order is used.
- The remaining winning articles are guaranteed this position in the following weeks, again in alphabetical order. No further voting should take place on these, which remain at the top of the table with notices to that effect. Further nominations and voting take place to determine future winning articles for the following weeks.
- Winning articles may be named New Draft of the Week beyond their last eligible date if their circumstances are so described above.
- The article with the most specialist supporters is used. Should this fail to produce a winner, the article appearing first by English alphabetical order is used.
Administrators
The Administrators of this program are the same as the admins for CZ:Article of the Week.
References
See Also
- CZ:Article of the Week
- CZ:Markup tags for partial transclusion of selected text in an article
- CZ:Monthly Write-a-Thon
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