Scalar & Vector Equations: What is the Difference?

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Discussion Overview

The discussion revolves around the differences between scalar and vector equations, particularly in the context of representing lines in two and three dimensions. Participants explore the implications of these equations and their geometric interpretations.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that scalar equations, like y=2x+3, generate collinear points, while vector equations generate vectors that are not collinear.
  • One participant questions how a vector equation, such as r = (2,1,3) + t(1,2,4), can be considered the "equation of a line."
  • Another participant emphasizes the physical meaning of the position vector r, stating it represents a spatial displacement from the origin and traces out a line.
  • Some participants propose that it is possible to produce a set of scalar equations that describe a line in R^3, providing an example of a line expressed in both scalar and vector forms.
  • There is mention of a parametric representation of lines in three dimensions, with a suggestion to look up "direction cosines" for further understanding.

Areas of Agreement / Disagreement

Participants express differing views on whether scalar equations can represent lines in three dimensions, with some asserting it is possible while others maintain it is not. The discussion remains unresolved regarding the implications of these representations.

Contextual Notes

Participants reference specific forms of equations and their geometric interpretations, but there are unresolved assumptions about the definitions and conditions under which these equations apply.

Saad
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Scalar equations such as y=2x+3 actually generate POINTS which are collinear. A vector equation, as the name implies, generates VECTORS, and these vectors are definitely NOT COLLINEAR.

How then can we say that an equation such as
r = (2,1,3) + t(1,2,4) is the "equation of a line"?

Also, why is it not possible to produce a scalar equation for a line in 3-D?
 
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Originally posted by Saad
Scalar equations such as y=2x+3 actually generate POINTS which are collinear. A vector equation, as the name implies, generates VECTORS, and these vectors are definitely NOT COLLINEAR.

How then can we say that an equation such as
r = (2,1,3) + t(1,2,4) is the "equation of a line"?

Also, why is it not possible to produce a scalar equation for a line in 3-D?

Consider the physical meaning of the vector r. That is called the position vector. It represents a spatial displacement from a point called the origin. The tip of the vector defines a point and it is that point we are referring to as the position.

Since r is the position vector which traces out a line, i.e. the tip of the vector traces out a line, the its called the equation of a line. Likewise the tip of the vector

[tex]\mathbf{r} = cos \theta \mathbf{i} + sin \theta \mathbf{j}[/tex]

traces out a circle. Therefore one can say that this is the equation of a circle.
 
It is possible to produce a set of scalar equations that generate a line in R^3

eg the line (1,2,3) + t(2,3,1) is also described as

(x- 1)/2 = (y-2)/3 = z-3


just as the original scalar equation you gave is expressible as a vector equation:

L = (0,3) +t(1,2)
 
Last edited:
matt grime said:
It is possible to produce a set of scalar equations that generate a line in R^3

eg the line (1,2,3) + t(2,3,1) is also described as

(x- 1)/2 = (y-2)/3 = z-3


just as the original scalar equation you gave is expressible as a vector equation:

L = (0,3) +t(1,2)

This is called a parametric rep, with t as the parameter. For three dimension look up "direction cosines".
 

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