Vector Products: What Happens When Professors Refuse to Explain?

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Discussion Overview

The discussion centers around the definition and understanding of vector products, particularly in the context of why a vector product is not generally defined pointwise or component-wise. Participants explore the implications of different definitions and the geometric significance of vector products.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions why a vector product is not defined pointwise, expressing frustration over a lack of explanation from their professor.
  • Another participant seeks clarification on the term 'pointwise' and the definition of 'vector product', suggesting that there may be multiple interpretations of these terms.
  • A participant defines a component-wise multiplication for vectors, indicating a specific operation that could be considered a form of vector multiplication.
  • Another participant acknowledges the definition provided but distinguishes it from the strict meaning of vector products, noting that the operation described does not have geometric significance.
  • One participant references a viewpoint that there is no need to introduce the component-wise product at certain levels, emphasizing the established definitions of scalar and vector products in \mathbb{R}^3.
  • Concerns are raised that the operation defined by one participant does not correspond to the traditional vector product, which relies on geometric interpretations involving angles and the right-hand rule.

Areas of Agreement / Disagreement

Participants express differing views on the definition and significance of vector products, with no consensus reached on the appropriateness of defining vector products pointwise or the relevance of the proposed component-wise multiplication.

Contextual Notes

There are unresolved assumptions regarding the definitions of vector products and the implications of different types of multiplication in vector spaces. The discussion highlights the need for clarity in terminology and the potential limitations of the proposed definitions.

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Why is a vector product not generally defined pointwise? My professor simply gave a mysterious "you'll see why later". What's the worst that can happen?
 
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What do you mean 'pointwise'? And for that matter what do you mean by 'a vector product', there is generally considered to be one such. Presumably he means something like 'because there is no nice formula for what v^w is in terms of v and w', though I'm not sure I agree with that. Maybe if you even gave what your definition is we might have some better idea.
 
By "pointwise" I mean "component-wise". Define x:VXV->V as (a,b,c,d)x(e,f,g,h)=(ae,bf,cg,dh) to be the vector multiplication on some vector space V.
 
Oh, right, so that's what you mean. Since vector product has a strict meaning wondered what you were up to, as does pointwise which is strictly different from what you've done: it literally is used to talk about things happening at one point at a time.

Well you can define a 'multiplication' like that, it is certainly a bilinear map from VxV to V that does a lot of interseting things, it even has a name that I can't recall (when applied to a vector space of matrices), and it makes V into an algebra. But it is however, at the level you're looking at useless: it does nothing geometric, or nice.
 
As matt grime has pointed out, there is at most levels no need to introduce this kind of a product.

Instead we have scalar and vector products defined (on [itex]\mathbb{R}^3[/itex]) as the products of magnitudes, the cosine/sine of the angle between them (and we make the vector product into a vector by the right-hand screw rule).

Then we can work out what to do with the components once we've got a definite basis. And neither of these correspond to the operation you've defined.
 

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