Proving Parallel Vectors: Cross Product Question Explained

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Homework Help Overview

The discussion revolves around proving that if the cross product of two vectors, \(\vec{a}\) and \(\vec{b}\), equals zero, then the vectors are parallel. The subject area is vector mathematics, specifically focusing on properties of the cross product in three-dimensional space.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the implications of the cross product being zero and discuss the geometric interpretation of parallel vectors. They raise questions about the relationships between the components of the vectors and whether these relationships indicate parallelism.

Discussion Status

Some participants have offered insights into the relationships between the components of the vectors, suggesting that these relationships may imply parallelism. There is ongoing exploration of the definitions and interpretations of the cross product and its implications for vector orientation.

Contextual Notes

Participants mention the need for clarity on the definitions of the zero vector and the conditions under which vectors are considered parallel. There is also a reference to the constraints of the problem, such as the assumption that the vectors are non-zero.

spoc21
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Homework Statement



Prove that If \vec{a}x \vec{b} = 0, then \vec{a}is parallel to \vec{b}.

Homework Equations


The Attempt at a Solution


I tried attempting the solution by using the following:

\vec{a} = [a1, a2, a3]
\vec{b} = [b1, b2, b3]When I took the cross product of a x b I got::

[a2b3 - b2a3 a3b1 - b3a1, a1b2 - b1a2]

and we can make this equal to 0, but I am confused here; I have no idea on how to prove that when a x b is 0, vectors a and b are parallel.

any help is appreciated!
 
Last edited:
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Imagine the zero as the zero vector, defined in R3 as 0=[0,0,0]

Edit: You could view the geometric interpretation. In which case you will have a parallellogram with area equal to zero.
 
Last edited:
thanks, so this is what I end up with:

a2b3 = b2a3...(1)

a3b1 = b3a1...(2)

a1b2 = b1a2...(3)

Does this suggest that the two vectors, a and b are parallel?
 
Bump..any one?

thanks
 
spoc21 said:
Bump..any one?

thanks

Its it simply


Let \vec{a},\vec{b} \neq 0 and let

\vec{a} \times \vec{b} = |\vec{a}| \cdot |\vec{b}| \cdot sin(\theta}) \cdot \mathrm{n} be the definition of the cross product where 0 \leq \theta \leq \pi

then for \theta = 0 \vec{a} \times \vec{b} = |\vec{a}| \cdot |\vec{b}| \cdot sin(0}) \cdot \mathrm{n} = 0

q.e.d.
 
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dot product? I don't understand it, could anyone please help..
 
Hi spoc21! :smile:
spoc21 said:
a2b3 = b2a3...(1)

a3b1 = b3a1...(2)

a1b2 = b1a2...(3)

Does this suggest that the two vectors, a and b are parallel?

Yes, because they become

a2/a3 = b2/b3...(1)

a3/a1 = b3/b1...(2)

a1/a2 = b1/b2...(3) :wink:
 
tiny-tim said:
Hi spoc21! :smile:Yes, because they become

a2/a3 = b2/b3...(1)

a3/a1 = b3/b1...(2)

a1/a2 = b1/b2...(3) :wink:

Thanks tiny-tim So basically if I write the vectors in this form, I am showing that they are parallel, and this would be enough for the proof? Also, just for my knowledge could you please elaborate on this..
Thank you very much :smile:
 
Hi spoc21! :smile:
spoc21 said:
Thanks tiny-tim So basically if I write the vectors in this form, I am showing that they are parallel, and this would be enough for the proof?

Yes, that's enough to do it. :wink:

To elaborate …two vectors are parallel if one is a scalar times the other …

and you can easily check that that means that the ratios of their coordinates must be the same. :smile:
 
  • #10
Thanks! :smile:
 

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