Vectors & Angles: Find Relationship between \alpha & \beta

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Discussion Overview

The discussion revolves around finding a relationship between the angles \(\alpha\) and \(\beta\) formed by three vectors \(a\), \(b\), and \(c\). The scope includes theoretical exploration of vector relationships, specifically focusing on geometric interpretations and mathematical formulations.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that the dot product can be used to express the angles \(\alpha\) and \(\beta\) in terms of the magnitudes of the vectors and their respective angles.
  • Another participant questions whether a relationship can be established without using the dot product, proposing the consideration of the vectors as edges of a parallelepiped.
  • A subsequent reply emphasizes that even when considering a parallelepiped, the concepts of dot and cross products appear to be essential for deriving any relationship.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of using dot products to find a relationship between the angles, indicating that there is no consensus on the approach to take.

Contextual Notes

The discussion highlights the dependence on mathematical definitions and the potential limitations of the proposed methods, particularly regarding the use of dot and cross products in vector analysis.

Apteronotus
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Hi,

Suppose you have three vectors a, b and c.
Say the angle between a and c is given by [tex]\alpha[/tex], and between b and c by [tex]\beta[/tex].

Can we find a relationship between [tex]\alpha[/tex] and [tex]\beta[/tex]?

Thanks in advance,
 
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Well, if you know dot products then you know:

[tex]a . c = ||a|| ||c|| cos(\alpha)[/tex]
[tex]b . c = ||b|| ||c|| cos(\beta)[/tex]

So you could rearrange that to find a relationship between [tex]\alpha[/tex] and [tex]\beta[/tex].
 
Thank you Whybother,

Of course you are correct. But I'm wondering can a relation be found that does not involve the dot product? Perhaps if we think of the three vectors as the edge of a parallelepiped?
 
Apteronotus said:
Thank you Whybother,

Of course you are correct. But I'm wondering can a relation be found that does not involve the dot product? Perhaps if we think of the three vectors as the edge of a parallelepiped?

Even if you are defining a parallelepiped in 3space, I don't think you can escape from the notion of dot and cross products. Looking at a http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Parallelepiped_volume.svg/780px-Parallelepiped_volume.svg.png" of it, it seems unavoidable to me.
 
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