vector multiplication

[jmmiller]

given the vector w = i+2j+5k
what is (-w)(w)

i j k i j
-1 -2 -5 -1 -2
1 2 5 1 2

-10i -5j-2k - (-5j-10i-2k) = 0. Is this correct?

 

[tkhunny]

You'll have to explain what is meant by the juxtaposition of two vectors.

It appears you are trying a Cross Product. If so, why would you bother to multiply it all out. Isn't there an important concept related to parallel vectors and cross products?

 

[pka]

given the vector w = i+2j+5k
what is (-w)(w)

Is it \(-\vec{w}{\Large\cdot}\vec{w}\) the dot product?
OR is it \(-\vec{w}{\large\times}\vec{w}\) the cross product?

 

[topsquark]

given the vector w = i+2j+5k
what is (-w)(w)

i j k i j
-1 -2 -5 -1 -2
1 2 5 1 2

-10i -5j-2k - (-5j-10i-2k) = 0. Is this correct?

Since you are writing the unit vectors I'm guessing that you are using the cross product. But you really should mention that.

If you are going to write it all out like that, then write it as a determinant:
[math]- \vec{w} \times \vec{w} = \left | \begin{matrix} \hat{i} & \hat{j} & \hat{k} \\ \text{-1} & -2 & -5 \\ 1 & 2 & 5 \end{matrix} \right |[/math]
I agree with tkhunny, though, this is very inefficient. You can easily get the answer using the magnitude of the cross product. In addition, think about what the cross product does. You can write the answer down just from that.

But, hey, it's always good to be able to do the problem using several different ways. So go for it!

-Dan

 

[jmmiller]

Is it \(-\vec{w}{\Large\cdot}\vec{w}\) the dot product?
OR is it \(-\vec{w}{\large\times}\vec{w}\) the cross product?

dot product

You'll have to explain what is meant by the juxtaposition of two vectors.

It appears you are trying a Cross Product. If so, why would you bother to multiply it all out. Isn't there an important concept related to parallel vectors and cross products?

correct, but that isn't the context the teacher is sking for answers in.

Is it \(-\vec{w}{\Large\cdot}\vec{w}\) the dot product?
OR is it \(-\vec{w}{\large\times}\vec{w}\) the cross product?

I need the dot product. I don't realize there is a difference. How do I do a dot product?

 

[pka]

dot product

correct, but that isn't the context the teacher is sking for answers in.

I need the dot product. I don't realize there is a difference. How do I do a dot product?

What you have is \(<-1.-2,-5>{\Large\cdot}<1,2,5>=(-1)(1)+(-2)(2)+(-5)(5)=~?\)

 

[Deleted member 4993]

dot product

correct, but that isn't the context the teacher is sking for answers in.

I need the dot product. I don't realize there is a difference. How do I do a dot product?

You have not been taught to dot product - yet you were given this assignment?

Please review at https://www.khanacademy.org/math/li...oducts/v/vector-dot-product-and-vector-length

If you do not understand some parts - please come back and tell us exactly where you are stuck.

 

[jmmiller]

given the vector w = i+2j+5k
what is (-w)(w)

i j k i j
-1 -2 -5 -1 -2
1 2 5 1 2

-10i -5j-2k - (-5j-10i-2k) = 0. Is this correct?

You have not been taught to dot product - yet you were given this assignment?

Please review at https://www.khanacademy.org/math/li...oducts/v/vector-dot-product-and-vector-length

If you do not understand some parts - please come back and tell us exactly where you are stuck.

Great! I've got it now. Trying to learn without a teacher is challenging!

 

[pka]

Trying to learn without a teacher is challenging!

Bad IDEA