What is straight lines: Definition and 44 Discussions

In geometry, a line is an infinitely long object with no width, depth, or curvature. Thus, lines are one-dimensional objects, though they may exist embedded in two, three, or higher dimension spaces. The word line may also refer to a line segment in everyday life, which has two points to denote its ends (endpoints). A line can be referred to by two points that lie on it (e.g.


{\textstyle {\overleftrightarrow {AB}}}
) or by a single letter (e.g.

{\displaystyle \ell }
Euclid described a line as "breadthless length" which "lies evenly with respect to the points on itself"; he introduced several postulates as basic unprovable properties from which he constructed all of geometry. Euclidean line and Euclidean geometry are terms introduced to avoid confusion with generalizations introduced since the end of the 19th century, such as non-Euclidean, projective and affine geometry.

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  1. M

    Number of lines equidistant from four points on a plane

    Hi, i'm trying to solve this problem. It's exercise 3 on page 5 from this book: Challenging mathematical problem with elementary solutions The solution is on page 41: I'm OK with the 4 circles in case 1: i can pick (inside/outside): ABC + D, ABD + C, ADC + B, BCD + A. What i cannot...
  2. Einstein44

    Triangle formed by straight lines

    I tried doing this problem, but couldn't find a solution that wouldn't involve a sketch... Finding the vertices was all I've got. Thanks in advance.
  3. L

    I In the Universe do straight lines exist?

    In the universe do straight lines exist? I know over long distances like interstellar and even shorter distances like between the Earth and around the gravity of the moon lines tend to curve, but do straight lines exist anywhere? Or just a desire for them to exist in nature if not for gravity...
  4. LCSphysicist

    Find the vector equation of the line that passes through the point P and intersects with the straight lines R and S

    I think that we can say that PPR = α*PRPS where PR and PS are the points where occurs the intersection on the line R and S. Obs: line r and s are found by knowing that the straight line intersection of two planes are n1 X n2 [cross product] Lr = (0,1,-2) + y(-1,1,1) Ls = (0,1,-1) + u(1,2,1)...
  5. R

    I Condition for a pair of straight lines

    While determining the condition for the pair of straight line equation ##ax^2+2hxy+by^2+2gx+2fy+c=0## or ##ax2+2(hy+g)x+(by^2+2fy+c)=0 ## (quadratic in x) ##x = \frac{-2(hy+g)}{2a} ± \frac{√((hy+g)^2-a(by^2+2fy+c))}{2a}## The terms inside square root need to be a perfect square and it is...
  6. pairofstrings

    B I need word problems about a pair of straight lines

    Hello, I want to see word problems on pair of straight lines to know the real-time applications. I want to find out what it means if a straight line intersects/is parallel to other straight line. Thanks!
  7. NaukowiecGirl

    Why do eyes only see in straight lines? (Light Physics)

    Hello! Please help me with this question. I'm currently studying Light and Geometric Optics and have become quite confused. When looking at diagrams of mirrors, and light relfection into eyes, why is it that eyes only see things when tracing the light in straight lines? For example, if an...
  8. arpon

    Angle between two straight lines

    Homework Statement Find the angle between the straight lines: ##(x^2+y^2)(\cos^2{\theta} \sin^2{\alpha} + \sin^2{\theta})=(x \tan{\theta} - y \sin{\alpha})^2## Homework Equations [Not applicable] The Attempt at a Solution Dividing by ##x^2##, ## (1+(\frac{y}{x})^2)(\cos^2{\theta}...
  9. O

    Greeks, Circles, Small Straight lines and Calculus

    Hello, I VAGUELY recall reading, some many years ago, a statement to the following... "The Greeks were obsessed with circles. Had they relaxed this obsession, they may have seen the significance of modeling curves with small straight lines, and thereby anticipated the Calculus." Is there any...
  10. T

    Non-Euclidean Physics & Straight Lines: Can They Coexist?

    Here's something I've been wondering about: does non-Euclidean modern physics imply that there are no straight lines in our universe? If so, how is this possible? With any circular object or space, one can always draw a straight line through it, right? Thanks.
  11. I

    Why do all objects move in straight lines?

    All objects not affected by gravity keep moving in a straight line for infinite...why?
  12. A

    Vector Calculus: Air flowing through loop of straight lines

    Homework Statement Air is flowing with a speed v in the direction (-1, -1, 1,) calculate the volume of air flowing through the loop consisting of straight lines joining (in order i presume) (1,1,0) (1,0,0) (0,0,0) (0,1,1) (1,1,0) Homework EquationsThe Attempt at a Solution I assume you have to...
  13. G

    Line integral of straight lines path (quick question)

    Homework Statement Hello, I have a quick question about the following problem F = (2y+3)i+xzj+(yz-x)k and straight lines from (0,0,0) to (0,0,1) to (0,1,1) to (2,1,1) Considering C1 is the line from (0,0,0) to (0,0,1) C2 is the line from (0,0,1) to (0,1,1) and C3 is the line from (0,1,1) to...
  14. adjacent

    The angle between two straight lines.

    Homework Statement Find the angle between two straight lines,##y=x## and ##y=2x##. Find the general formula for finding the angles. Homework Equations ##f(x)=mx+c## ##\tan(\theta)=\frac{opposite}{adjacent}## The Attempt at a Solution First I assign a value for x and calculate the...
  15. U

    Straight lines passing through fixed point

    Homework Statement If all the lines given by the equation (3\sin \theta + 5\cos \theta )x+(7\sin \theta - 3\cos \theta )y+11(\sin \theta - \cos \theta)=0 pass through a fixed point (a,b) forall theta in R then |a-b|= Homework Equations The Attempt at a Solution Dividing both sides by...
  16. B

    Straight lines and flat surfaces

    Suppose I have a parameterized line ##\phi:\mathbb{R}\to\mathbb{R}^n## given by ##\phi(t) = (x^\mu(t))|_{\mu=1}^n##. How can I tell that the line is straight. My best answer so far is that at every time ##t## the acceleration (2nd derivative) is parallel to the velocity (1st derivative), i.e...
  17. S

    MHB Homework: Bisectors and Intersection Points of Straight Lines

    Show that one of the bisectors of the angles between the pair of straight lines ax2+2hxy+by2=0 will pass through the point of intersection of the straight lines ax2+2hxy+by2+2gx+2fy+c=0 if h(g2-f2)=fg(a-b) Please help
  18. S

    MHB What is the area of the parallelogram enclosed by a pair of straight lines?

    If each of the equations ax^2+2hxy+by^2+2gx+2fy+c=0 and ax^2+2hxy+by^2-2gx-2fy+c=0 represents a pair of straight lines , find the area of the parallelogram enclosed by them . Please help
  19. Fredrik

    Linearity vs. takes straight lines to straight lines

    Linearity vs. "takes straight lines to straight lines" Homework Statement Prove that if ##\Lambda:\mathbb R^n\to\mathbb R^n## is a bijection that takes straight lines to straight lines, and is such that ##\Lambda(0)=0##, then ##\Lambda## is linear. Homework Equations Fock's theorem implies...
  20. X

    Solving Straight Lines Homework: Find Intersection Point

    Homework Statement Lines li: ix+(i+1)y+(i+2)=0; i=1,2 intersect at___ point? Homework Equations The equation is required at first! The Attempt at a Solution I am confused! How and where should I replace the values of i with 1 or 2? This is the first step but i am unable to do...
  21. K

    Finding the Area of a Triangle Inside a Circle and Parallel Line: How to Solve?

    1.There is a circle with the equation x^2 + y^2 - 2ax = 0. A line is drawn through the centre of the circle which is parallel to the line x+2y=0. and also intersects the circle at A and B. Find the area of the triangle AOB. My attempt- I calculated the slope of the given line(-1/2).So the...
  22. K

    How Do You Prove Point Equidistance and Find Loci in Geometry?

    1. A point P(x,y) is given equidistant from the points A(a+b,b-a) and B(a-b,a+b), then prove that bx = ay also find the locus of the variable point Z(a cos (theta), b sin (theta)), where (theta) is a variable quantity. 2. T0 prove that ax = by 3. In an attempt towards...
  23. T

    Linear Algebra- Finding the intersection of two straight lines.

    Homework Statement Determine whether the following two lines intersect: (x-2)/2 = (y+3)/1 = (z-4)/-3 ,and (x+3)/4 = (y+4)/1 = (-z+8)/4 Find an intersection point, then find the distance between the lines. Homework Equations Symmetric equations of a...
  24. Saitama

    What Are the Possible Loci When d (x, y) = max {|x|,|y|}?

    Homework Statement If the distance of any point (x,y) from the origin is defined as d (x, y) = max {|x|,|y|}, d (x, y) = a non zero constant, then the locus is (a) a circle (b) a straight line (c) a square (d) a triangle Homework Equations The Attempt at a Solution I don't...
  25. D

    QED and light traveling in straight lines

    I tried asking a similar question earlier, but based on the answers I got I wasn't able to convey myself well. Here's Feynman's argument (at least as I understand it) for why, from a Quantum perspective, light (roughly) travels in straight lines from points A to B: 1) the photon will travel...
  26. M

    Cutting a rectangle with straight lines

    Hi to everyone. I'm developing a puzzle game which involves some concepts of geometry. Suppose you have a rectangle formed by points A, B, C and D. Now, I add points X1 (lies on line AB), X2 (inside the rectangle) and X3 (lies on line AC). I'd like the output to be the two rectangles...
  27. S

    Forces, vectors and straight lines

    Where is it writ that forces must act in straight lines?
  28. S

    Geodesics and straight lines on a surface

    Homework Statement Let \gamma be a stright line in a surface M. Prove \gamma is a geodeisc The Attempt at a Solution In a plane we know a straight line is the shortest distance between two point. I am not sure if this applies to straight lines on a surface. Further more, there...
  29. J

    Do photons travel in straight lines?

    If a photon is detected at point B having been emitted at point A, can one deduce with near certainty that it must have traveled continuously in a straight line from A to B at velocity c?
  30. T

    Straight lines as shortest connections

    Homework Statement http://img193.imageshack.us/img193/1066/65157866.png The Attempt at a Solution Since v is a constant vector, I took it out the front of the integral and used the ftc so the integral of the derivative collapses to γ(t) with terminals a to b. Evaluating leads to γ(b) -...
  31. T

    Photons do not travel in straight lines

    I've been thinking about this for some time now and I wanted to get some feedback on it. If photons are supposed to travel along straight lines (for the sake of simplicity let's neglect the curvature of space-time), then how could one photon possibly be used in a double slit experiment? More...
  32. A

    Idea that EM radiation propagates through space in straight lines

    OK so I'm familiar with the idea that EM radiation propagates through space in straight lines at a uniform speed, that of light. The idea that light moves in waves, that each wavelength carries a uniform energy and that the wavelength differs yet because the same speed is maintained each...
  33. T

    What Is the Use of Families of Straight Lines in Geometry?

    This is my second post on PF =) I want to ask what is the use of Families of straight lines? I am thinking of A Family of Straight Lines Passing Through the Intersection of Two Lines. We have the equation: L1+kL2=0 where L1=L2=0 and k is a variable, right? But is it said that L2 is not...
  34. L

    Calculating Eccenticity of a Pair of Straight Lines

    how do we define the eccenticity of a pair of straight lines
  35. L

    Finding the Differential Equation for Straight Lines in Polar Coordinates

    Homework Statement Problem from Arnold's "Mathematical Methods of Classical Mechanics" on page 59. Find the differential equation for the family of all straight lines in the plane in polar coordinates. Homework Equations \Phi=\displaystyle\int^{t_2}_{t_1}...
  36. S

    Straight lines and complex numbers

    can anyone give me a detailed explanation on how to derive equation for a straight line, which is made up of points, each point representing a complex number..// pls help
  37. D

    Why Straight Lines? Uncovering the Theory Behind f(x,y)=0

    Given the real function f(x,y) = x^2 - 4xy + 3y^2, the equation f(x,y) = 0 shows in a graph as 2 straight lines, y=x and y=x/3. For pairs (x,y) between the lines, f(x,y) < 0; for (x,y) outside the lines, f(x,y) > 0. It is easy to prove the above, by substituting y=mx in the equation f(x,y)=0...
  38. M

    Demonstrating Tilted, Straight Lines in Rotating Sphere Spectra

    Homework Statement How do you show that in spectra taken along the equator of a rotating rigid sphere the spectral lines are tilted but straight? Homework Equations There were no equations given in the problem, so I guess you can use whatever equations you know. The Attempt at a...
  39. Mk

    Hour & Minute Hand: Straight Lines & Overlaps

    How many times per day (00:00-24:00) do the hour and minute hand form a straight line together? Would this be the same for when they overlap? Say, the minute hand goes pi/1 hr and the hour hand goes (pi/12)/1 hr. Then what?
  40. P

    Transformations taking straight lines to straight lines

    On R^n, I'd say the only smooth transformations taking straight lines to straight lines are the affine transformations. Would I be right saying that?:smile: How would one go about proving that?
  41. Nim

    How Does Curved Spacetime Influence the Path of Light and Objects in Space?

    The explanation I've heard for gravity as depicted by relativity, is that it's the consequence of mass curving spacetime. This means that the reason why the light from a star behind Sun is bent is because evening though the light is traveling a straight line, since space itself is bent...
  42. H

    Contour Integral with just straight lines?

    I'm trying to solve this contour integral shown on the attached file, I know usually that they involve curved lines. I know that this is trivial but I need some help with the problem. Please take a look.
  43. T

    Does straight lines have slopes of tanget?

    Does straight lines have slopes of tangent? Does straight lines have slopes of tangent?.. :confused: i had no clue when this question came on a test..so i just left it...do they?..i just thought maybe they didn't since they have no limit. Hope i get an answer for this question. Tanya
  44. O

    Why thunderbolts don't travel in straight lines

    Why thunderbolts don't travel in straight lines ? I think the electrical charge likes to move in the shortest path to the Earth and the shortest path must be a straight line !