How to specify the direction of an area vector?

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Discussion Overview

The discussion revolves around the specification of the direction of an area vector for a triangle defined by two consecutive sides, represented by vectors ##\vec{a}## and ##\vec{b}##. Participants explore the implications of using different cross products and the conventions involved in determining the direction of the area vector.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the area of the triangle is given by ##\frac{1}{2} | \vec{a} \times \vec{b} |## and question how to specify the direction of the area vector.
  • One participant states that the direction is determined by the right-hand rule, where the thumb points in the direction of ##\vec{a}##, the pointer in the direction of ##\vec{b}##, and the middle finger indicates the direction of ##\vec{a} \times \vec{b}##.
  • Another participant questions why the area is associated with ##\vec{a} \times \vec{b}## rather than ##\vec{b} \times \vec{a}##, highlighting that the two cross products yield opposite directions.
  • A further response suggests that the choice of direction is a convention, comparing it to other mathematical conventions, and argues that the chosen convention aligns with the formula involving determinants.

Areas of Agreement / Disagreement

Participants express differing views on the choice of direction for the area vector, with some supporting the right-hand rule and others questioning the convention of using ##\vec{a} \times \vec{b}## over ##\vec{b} \times \vec{a}##. No consensus is reached regarding the necessity or implications of this choice.

Contextual Notes

The discussion highlights the dependence on conventions in vector mathematics and the implications of choosing one cross product over another, but does not resolve the underlying questions about these conventions.

Hawkingo
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We all know that the area of a triangle having consecutive sides as ##\vec { a }## and ##\vec { b }## has the area ##\frac { 1 } { 2 } | \vec { a } \times \vec { b } |## but what is the direction of that area vector? I mean if we consider ##\vec { a } \times \vec { b }## that will be one direction and if we consider ##\vec { b } \times \vec { a }## then that will be the opposite direction but as we know an vector always has a particular direction so how to specify the direction of the area vector in this case?
 
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Hawkingo said:
We all know that the area of a triangle having consecutive sides as ##\vec { a }## and ##\vec { b }## has the area ##\frac { 1 } { 2 } | \vec { a } \times \vec { b } |## but what is the direction of that area vector? I mean if we consider ##\vec { a } \times \vec { b }## that will be one direction and if we consider ##\vec { b } \times \vec { a }## then that will be the opposite direction but as we know an vector always has a particular direction so how to specify the direction of the area vector in this case?
The direction is determined by the right hand rule: thumb ##\vec{a}##, pointer ##\vec{b}##, middle ##\vec{a}\times \vec{b}##.
 
fresh_42 said:
The direction is determined by the right hand rule: thumb ##\vec{a}##, pointer ##\vec{b}##, middle ##\vec{a}\times \vec{b}##.
I know but I want to ask that why consider ##\vec{a}\times \vec{b}## for the area of the triangle but not ##\vec{b}\times \vec{a}## ? The 2 cross products have different directions.
 
Hawkingo said:
I know but I want to ask that why consider ##\vec{a}\times \vec{b}## for the area of the triangle but not ##\vec{b}\times \vec{a}## ? The 2 cross products have different directions.
Like with all things which can be oriented: make your choice! Why do we write debts as negative numbers and not the other way around? Why do we define ##\int_a^b f(x)dx = F(b)-F(a)## and not the other way around? It's only a convention, and in this case I find it suited compared with the formula behind: ##(\vec{a}\times \vec{b})_1=+ \det\left(\begin{bmatrix}a_2& b_2\\a_3&b_3\end{bmatrix} \right)##, i.e. to start with a positive sign.
 
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