Is perpendicularity transitive?

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Discussion Overview

The discussion centers on whether the relation of perpendicularity is transitive among straight lines, exploring both theoretical and practical implications. Participants examine the conditions under which perpendicularity may or may not be considered transitive, particularly in different dimensions and contexts.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the transitivity of perpendicularity, suggesting that three mutually perpendicular lines could imply transitivity.
  • Another participant clarifies that if line a is perpendicular to b and b is perpendicular to c, it does not necessarily imply that a is perpendicular to c for all lines.
  • A different viewpoint emphasizes that mutual perpendicularity is a specific case and questions whether conditions like coplanarity affect the transitivity of perpendicularity.
  • One participant argues that in the plane (R2), perpendicularity is not transitive, providing an example where L1 is perpendicular to L2 and L2 is perpendicular to L3, but L1 is parallel to L3.
  • A counterexample using vectors in three dimensions is presented, showing that while two vectors can be perpendicular to a third, they may not be perpendicular to each other, thus concluding that perpendicularity is not transitive.

Areas of Agreement / Disagreement

Participants express differing views on the transitivity of perpendicularity, with some arguing against it in general contexts while others suggest specific cases where it might hold. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants mention conditions such as coplanarity and dimensionality, indicating that the transitivity of perpendicularity may depend on these factors. The discussion does not reach a consensus on the implications of these conditions.

PhysicoRaj
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Is perpendicularity transitive??

Is the relation 'perpendicularity' transitive on the set of straight lines? I've been taught 'NO', but I think yes because three mutually perpendicular lines prove the same.
Thanks..
 
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If perpendicularity is transitive this means that if line a is perpendicular to b and b is perpendicular to c, that this implies a is perpendicular to c for ALL lines a,b,c not only for some lines.
 
So, mutual perpendicularity is the only case which holds good, and hence the relation is not transitive on the whole. I get it. But is there any condition such as coplanarity mentioned in the rule?
 
PhysicoRaj said:
Is the relation 'perpendicularity' transitive on the set of straight lines? I've been taught 'NO', but I think yes because three mutually perpendicular lines prove the same.
What does "three mutually perpendicular lines prove the same." mean?

If we're talking about lines in the plane (R2), then perpendicularity is NOT transitive. Suppose L1 ##\perp## L2, and that L2 ##\perp## L3. Then clearly, L1 || L3, so transitivity fails.
 
Lets consider vectors to represent our lines. (in three dimensions)

Let A = [1,0,0], B = [0,1,0], and C = [1,0,0].

Since the dot product of A and B gives us 0, we know that A and B are perpendicular. [1]
Similarly, B and C are perpendicular.
However, the dot product of A and C gives us 1, hence, A and C are not perpendicular.

This counterexample allows us to conclude that the proposition "Perpendicularity is transitive" is false.

[1] http://en.wikipedia.org/wiki/Dot_product#Properties (property 5)This is precisely the same argument that Mark44 used except we fixed the third component at zero. We can further generalize this to n dimensions by fixing components three to n at zero.
 
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