How to Prove Distributive Laws for 3D Vector Cross Products?

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Discussion Overview

The discussion centers on proving the distributive laws for 3D vector cross products, specifically the expressions \( p \times (q + r) = p \times q + p \times r \) and \( p \times (q \times r) = (p \times q) \times r \). The scope includes mathematical reasoning and homework-related inquiries.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asks for proof of the distributive law for the cross product.
  • Another participant requests clarification on the definition of the cross product.
  • There is a suggestion that the problem may be homework-related.
  • One participant attempts to expand the left-hand side of the first equation but expresses uncertainty about the next steps.
  • Another participant emphasizes the importance of correctly using notation in vector expressions.
  • A participant states that the cross product is not associative, questioning the validity of the second expression.
  • Another participant agrees that the equality \( p \times (q \times r) = (p \times q) \times r \) holds only under specific conditions and refers to the vector triple product.
  • One participant provides a specific case where the second expression does not hold true by substituting \( q = p \).
  • There is a request for further clarification on what the distributive law entails.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the second expression, with some asserting it is not generally true while others seek to clarify the distributive property of the cross product. The discussion remains unresolved regarding the conditions under which the second expression may hold.

Contextual Notes

Participants have not fully established the mathematical steps required to prove the distributive laws, and there are unresolved questions about the definitions and properties of the cross product.

chocbizkt
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2 questions i have;

1. proove that; p x ( q + r ) = p x q + p x r

2. and p x ( q x r ) = ( p x q ) x r

where;

p = p1i + p2j + p3k
q = q1i + q2j + q3k
r = r1i + r2j + r3k
 
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Well, start with telling us what you know. What is the cross product between two vectors?
 
Sounds like homework.

Do you know the result of these expressions: i x i , i x j, etc... , ?
 
yes well so far for Qn1.
i have left handside p x (q + r) = (p1i + p2j + p3k)( (q1+ r1)i + (q2+ r2)j + (q3+ r3)k)

am i on the right track?
 
p x (q + r) = (p1i + p2j + p3k) x ( (q1+ r1)i + (q2+ r2)j + (q3+ r3)k)
Don't forget these important symbols.
 
thanks i don't know where to go from there
 
chocbizkt said:
thanks i don't know where to go from there

Can you now explicitly calculate the cross-product of two vectors?
 
Cross product is not associative, so i don't see how

px(qxr) = (pxq)xr
 
SeReNiTy is correct. \vec p\times(\vec q \times\vec r) = \vec (p\times\vec q) \times\vect only under some special circumstances. Tthe two forms are not equal in general. Google "vector triple product".
 
  • #10
true, i found out; px(qxr) = (pxq)xr
if you let q=p

then px(pxr)=(pxp)xr

such that (pxp)=0

therefore its not true statement
 
  • #11
chocbizkt said:
yes well so far for Qn1.
i have left handside p x (q + r) = (p1i + p2j + p3k) x ( (q1+ r1)i + (q2+ r2)j + (q3+ r3)k)

am i on the right track?

Just grind through that. By that I mean set up the determinant and perform the algebra.
 
  • #12
SeReNiTy said:
Cross product is not associative, so i don't see how

px(qxr) = (pxq)xr


Then wat is distributive law?:confused: :confused:

Vinodh
 
  • #13
Vinodh said:
Then wat is distributive law?:confused: :confused:

Vinodh

Number 1. in the original post is the distributivity (under addition) of the cross product: i.e. ax(b+c)=axb +axc
 

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